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Page      5,  1 

ne    2, 

€rrata 

For  enervate  read  innervate. 

Page    40,  1 

ne  36. 

For  inferior  read  superior. 

Page    41,  I 

ne    5. 

For  upward  read  downward. 

Page    57,  1 

ne  16. 

For  across  read  along. 

Page    80,  li 

ne  15. 

For  hyperopia  read  myopia. 

Page    80,  1 

ne  20. 

For  Bupthaimos  read  Buphthalmos. 

Page  124,  li 

ne  29. 

For  radius  read  diameter. 

Page  320,  1 

ne  23. 

For  unity  read  uterque. 

Page  321,  1 

ine    7. 

For  sac  read  duct. 

Page  319,  1 

ne  10. 

For  temporal  read  occipital. 

Page  414,  1 

ine    2. 

For  across  read  along. 

Page  414,  1 

ne  15. 

For  180  read  90. 

OCULO-  REFRACTIVE 
CYCLOPEDIA 

and 

DICTIONARY 

By 
THOMAS  G.  ATKINSON,  M.  D.,  B.  Sc. 

FORMERLY      PROFESSOR      OF      PHYSIOLOGY      AND      NEUROLOGY,      CHICAGO 
COLLEGE     OF     MEDICINE     AND     SURGERY,      CHICAGO     COLLEGE      OF 
DENTAL  SURGERY;   AND   THE   AMERICAN   MEDICAL   COLLEGE, 
ST.   LOUIS,   MO.      EDITOR  OF  THE   MEDICAL  STANDARD, 
CHICAGO,  AND  THE  MEDICAL  BRIEF,  ST.  LOUIS. 
AUTHOR    OF    "ESSENTIALS    OF    REFRAC- 
TION,"      "FUNCTIONAL       DIAG- 
NOSIS,"   ETC. 


UNABRIDGED    AND    FULLY    TI-LUSTRATED 


Published    by 

THE      PROFESSIONAL      PRESS,      Inc 

CHICAGO 


I 


r 


Copyright,   1921 


The   Professional    Press,    Ine. 


Chicago,  U.  S.  A. 


A7 

Authors  roretvord  -^ 


For  several  years  the  author  has  had  in  mind,  and  has  been 
gradually  working  toward,  the  preparation  of  such  a  work  as 
this,  from  various  points  of  view.  Every  applied  science 
demands  an  encyclopedia  and  dictionary  which  crystallizes  its 
subject-matter  and  standardizes  its  nomenclature.  Ocular 
refraction,  which  in  the  last  twenty-five  years  has  developed 
into  the  fullness  and  dignity  of  an  independent  applied  science, 
had  no  such  representation.  \\^hat  was  still  more  important,  as 
ocular  refraction  became  more  and  more  voluminous  and  many- 
sided,  it  seemed  ver}'  desirable  that  its  various  phases  should 
all  be  assembled  in  co-ordinated  form,  between  the  covers  of  a 
single  book,  where  the  whole  subject  could  be  conveniently  and 
comprehensively  studied.  A  complete,  unabridged  encyclopedia 
and  dictionary  fulfills  both  of  these  desiderata. 

It  takes  two  sets  of  forces,  however,  to  produce  a  worth- 
Avhile  book,  viz.,  authorship  and  publication ;  and  this  dream  of 
the  author's  would  have  remained  a  barren,  half-realized  dream, 
of  no  practical  service  to  the  profession,  if  it  had  not  met  its 
necessary  complement.  It  fortunately  happened  that  another 
man  cherished  the  same  vision,  and  possessed  the  capacity,  the 
experience,  the  energy  and  the  organization  to  give  it  tangible, 
concrete  form.  That  man  was  the  president  of  the  Professional 
Press,  through  Avhose  efificient  instrumentality  the  undertaking 
now  emerges  as  an  accomplished  fact. 

In  the  preparation  of  the  book  both  author  and  publisher 
have  held  steadily  before  them  two  paramount  aims:  (1)  That 
it  should  completely  cover  the  ground,  so  that,  as  the  publisher 
frequently  expressed  it  in  his  cogent  way,  "Nobody,  looking  in 
it  for  whatever  information,  should  have  to  close  the  covers  and 
say  'It  isn't  there'.''  (2)  That  it  should  be  written  and  arranged 
in  such  a  way  as  to  be  of  maximum  usefulness  to  the  practising 
refractionist  with  a  minimum  of  difficulty  or  trouble.  Every 
step  of  the  work  has  been  taken  with  these  two  purposes  in  view. 
How  successfully  they  have  been  achieved,  the  reader  must 
judge. 

Another  consideration  was  kept  in  mind,  which  might  or 
might  not  have  contributed  to  the  main  purposes  as  set  forth 

^675144 


above,  according  to  the  care  and  skill  with  which  it  was  handled, 
namely,  that  the  text  should  be  as  concise  as  possible  without 
impairing  its  effecti\eness,  avoiding  unnecessary  repetition  and 
prolixity  of  language.  In  the  carrying  out  of  this  consideration, 
the  various  phases  of  the  subject  have  been  thoroughly  dis- 
cussed, in  every  instance,  under  the  major  heading  to  which  they 
respectively  belong;  minor  headings  are  Ijriefly  defined,  and  a 
cross-reference  given  in  each  case,  to  the  major  heading  under 
which  a  full  account  may  be  found. 

It  has  been  impossible,  of  course,  to  prepare  a  work  of  this 
character  without  having  frequent  recourse  to  consultation  and 
comparison  of  many  text-books  and  works  of  reference,  too 
numerous  to  mention.  Where  any  of  these  have  been  directly 
cited,  proper  credit  is  given  in  the  text.  Special  obligation  must 
here  be  expressed  to  Dr.  Chas.  Sheard,  Physiologic  Opticist  to 
the  American  Optical  Company,  and  Editor  of  the  Am.erican 
Journal  of  Physiologic  Optics,  for  the  entire  article  on  "Ophthal- 
mometry'";  to  Robert  D.  Pettet  for  the  section  on  "Frame  Fit- 
ting" ;  and  to  the  artists  and  printers  who  have  so  efficiently 
cooperated  to  make  a  finished  book. 

T.  G.  A. 
Chicago,  III. 
August  i8,  1921. 


Librs^  y  cf  the  Alamerta 

oounuy        oaiation 

of  Optt.metrists 

OCULO -REFRACTIVE 
CYCLOPEDIA  AND    DICTIONARY 

Abducens.  A  descriptive  name  given  to  the  sixth  pair  of  cra- 
nial nerves,  because  their  function  is  to  enervate  the  external 
rectus  muscles  and  turn  the  eyes  outward.  The  term  is  also 
sometimes  applied,  but  wrongly,  to  the  external  rectus  muscle 
itself.    For  detailed  account  of  the  abducens,  see  Nerves. 

Abduction.  The  turning  of  the  eyes  outward  beyond  the  median 
line  by  the  external  rectus  muscles.  The  effect  is  known  as 
negative  convergence.  It  can  be  performed  by  the  external 
recti  alone,  and  whether  the  obliques  ordinarily  aid  or  not  is 
not  known.  It  is  regarded  as  an  involuntary  function ;  but 
under  the  stimulus  of  prisms  (base  in),  and  the  desire  for  a 
single  image,  it  can  be  forced. 

Power  of  abduction  is  expressed  in  terms  of  prism  dioptres, 
the  deviation  of  the  visual  axis  from  the  median  line  being 
considered  similarly  to  the  deviation  of  a  ray  of  light  by  a 
prism.  It  may  be  measured  by  finding  the  strongest  prism, 
base  in,  with  which  the  eyes  can  maintain  single  vision. 
Normally,  this  amounts  to  8  or  10  prism  dioptres. 

The  word  is  also  applied  to  the  exercises  of  the  external 
rectus  muscles  by  means  of  graded  or  rotary  prisms,  base  in. 
The  value  of  such  exercises  is  questionable.  (See  Convergence.) 

Abductor.  A  general  term  applied  to  any  muscle  which  turns 
the  part  to  which  it  is  attached  outward  from  the  median  line. 
In  opthalmology,  the  external  rectus  muscle  of  the  eye. 

Aberration.  In  optics  this  term  is  specifically  applied  to  the 
failure  of  refracted  rays  of  light  to  find  a  common  focus.  There 
are  two  forms  of  aberration  recognized,  namely,  chromatic 
and  monochromatic  or  spherical. 

(1)  Chromatic  aberration  is  the  non-uniform  focussing  of 
the  different  color  waves  in  a  pencil  of  light,  due  to  the  dif- 


ABERRATION 

ferent  refrangibility  of  these  waves,  the  short  violet  waves 
being  refracted  most,  the  long  red  waves  least,  and  intermediate 
waves  in  proportional  degrees.  Chromatic  aberration  is  greatly 
accentuated  in  prism  refraction,  and  this  is  responsible  for  the 
separation  of  the  color  waves  in  the  spectroscope.  (See 
Spectrum.)  It  is  also  made  the  basis  for  testing  the  refraction 
of  the  human  eye.     (See  Chromatic  Test.) 

(2)  Monochromatic,  or  spherical,  aberration  is  the  failure 
of  a  spherical  refracting  body  properly  and  uniformly  to  focus 
rays  of  light  which  fall  upon  its  surface  at  certain  angles.  The 
strict  truth  of  the  matter  is  that  all  refracted  rays  are  aberrant. 
The  theory  of  object  and  image  points  (Gaussian  theory)  holds 
good  only  so  long  as  the  angles  made  with  the  principal  axis 
are  infinitesimally  small,  with  infinitesimal  objects,  images, 
and  thicknesses.  All  images  produced  by  uncorrected  systems 
are  ill-defined  and  blurred,  if  the  apertures,  or  fields  of  view, 
exceed  certain  limits.  The  wider  the  apertures,  i.  e.,  the  larger 
the  angles  made  with  the  axis,  the  greater  the  aberration. 
Hence  spherical  aberration  is  greatest  at  the  periphery  of  the 
spherical  surface. 

There  aie  four  principal  phases  of  monochromatic  aberra- 
tion: 

(1)  Aberration  of  axial  points. 

(2)  Aberration  of  elements,  i.  e.,  smallest  objects  at  right 
angles  to  the  axis. 

(3)  Aberration  of  lateral  object-points. 

(4)  Distortion  of  the  image. 

Astigmatism  is  an  example  of  the  last  two  phases  of  aberra- 
tion. 

The  human  eye  experiences  both  these  forms  of  aberration ; 
however,  «.o  far  as  spherical  aberration  is  concerned,  the  iris 
cuts  ofif  these  aberrant  waves  and  prevents  them  from  enter- 
ing; and  chromatic  alierration  is  normally  so  slight  as  to  be 
negligible.  The  same  holds  good  for  the  aberration  of  lenses 
used  in  front  of  the  eyes.  In  instruments  of  great  precision, 
as  microscopes,  telescopes,  etc.,  aberration  is  largely  overcome 
by  a  special  combination  of  substances  in  the  construction  of 
the  lenses.     (See  Achromatism.) 


ABSCISSA  7 

Abscissa.  In  optics,  a  dimension  used  to  designate  the  position 
of  the  point  where  a  ray  of  light  crosses  the  principal  axis 
with  respect  to  the  vertex  of  the  reflecting  or  refracting  sur- 
face as  an  origin.  For  reflection  Vj  =  — v,  where  v^  is  the 
distance  in  front,  and  v  the  distance  behind  the  plane  mirror. 
For  refraction,  where  u  is  the  distance  in  front  of  the  lens,  v 
the  distance  behind,  n  the  index  of  the  first  medium  and  n^  the 
index  of  the  second  medium,  then 

Hi 

V  =  —  X  u 
n 

Absolute.  The  word  has  various  technical  meanings  in  optics, 
all  of  which,  however,  have  the  common  idea  of  an  extreme 
limit. 

Absolute  hyperopia  is  a  degree  of  hyperopia  that  cannot  l)e 
compensated  for  at  infinity  by  means  of  accommodation. 

Absolute  index  of  refraction  is  the  power  of  a  medium  to 
refract  waves  which  enter  it  from  a  vacuum. 

Absorption.  As  applied  to  optics  this  term  refers  to  light  waves 
which  enter  a  body  or  substance  and  do  not  emerge  from  it, 
being  stopped  or  transformed  into  some  other  form  of  energy 
during  their  passage.     Such  waves  are  said  to  be  absorbed. 

All  substances  absorb  light  to  a  more  or  less  degree,  and 
it  is  the  relative  degree  of  absorption  for  different  radiations 
which  gives  to  various  substances  their  distinctive  qualities 
of  visibility  and  color.  A  body  which  should  absorb  all  light 
would  be  a  "perfectly  black  body,"  but  no  such  body  actually 
exists.  Most  bodies  show  a  selective  power,  readily  absorbing 
certain  wave-lengths  and  others  not  so  readily. 

The  law  which  governs  absorption  is  simple.  Whatever 
percentage  of  the  original  intensity  of  the  light  is  absorbed 
in  the  first  unit  of  penetration,  the  same  percentage  is  ab- 
sorbed in  the  second  unit,  and  so  on.  Thus,  the  original  in- 
tensity of  light  decreases  by  a  continuous  arithmetical  pro- 
gression, according  to  the  power  of  absorption  of  the  body  for 
this  particular  radiation.  If  I  stands  for  the  intensity  at  any 
given  depth  of  penetration,  lo  for  the  original  intensity,  j  for 


8  ACCOMMODATION 

the  amount  of  absorption  in  the  unit  of  penetration,  and  t  for 
the  unit  of  jx'netration.  then 

I=^To(l-j)' 
"Fhe  (|uestion  of  absorption  by  ophthahnic  lenses  is  an  im- 
portant one.  In  the  main,  lenses  should  be  as  transparent  as 
possible,  i.  e.,  they  should  absorb  as  little  as  possible  of  the 
waves  which  have  to  do  with  visibility.  It  is  often  desirable, 
however,  to  keep  certain  waves  from  'reaching  the  eye.  e.  g.. 
the  actinic  and  violet  waves,  and  special  forms  of  lens  are 
made  whose  composition  absorbs  these  undesirable  waves. 
Amber  lenses  are  of  this  character.  Smoked  lenses  exercise  a 
rather  uniform  absorption  for  all  light  waves,  and  thus  serve 
to  cut  down  the  total  amount  of  light  entering  the  eye. 

Accommodation. 

MECHANISM  OF  ACCOMMODATION. 

The  aspects  of  accommodation  as  a  physiologic  function, 
including  a  description  of  the  nervous  paths  by  which  it  is 
mediated,  will  be  found  in  the  section  on  Physiology  of  Vision. 
This  discussion  of  the  mechanism  of  acconimodalion  will  be 
restricted  to  the  purely  mechanical  side  of  the  subject. 

We  know  that  accommodation  is  ultimately  attained  by  in- 
creasing the  convexity  of  the  crystalline  lens,  principally,  if 
not  altogether,  of  the  anterior  surface  of  the  lens,  and  that 
this  is  dependent  upon  the  elasticity  of  the  lens  and  its  capsule. 
That  this  change  in  the  shape  of  the  lens  actually  takes  place 
is  readily  demonstrable  by  a  very  simple  experiment. 

If  a  lighted  candle  be  held  a  foot  or  so  from  the  eye  of  a 
person  who  is  gazing  into  the  distance,  a  little  to  one  side,  so 
as  to  form  an  angle  of  about  30  degrees  with  the  median  line, 
and  the  observer's  eye  be  placed  at  the  same  distances  on  the 
other  side  of  the  median  line,  three  images  of  the  candle  will 
be  seen  reflected  from  the  subject's  eye:  (1)  an  upright  image 
formed  by  the  cornea,  (2)  a  second,  larger  upright  image, 
formed  by  the  anterior  surface  of  the  lens,  and  (.3)  a  smaller, 
inverted  image  formed  by  the  posterior  surface  of  the  lens. 

If,  now,  the  subject  be  directed  to  look  at  a  point  close  to 
his  eyes,  images  1  and  3  remain  tuichanged  in  shape,  size  and 


ACCOMMODATION  9 

position,  but  image  2,  which  is  formed  by  the  anterior  surface 
of  the  lens,  becomes  smaller,  more  distinct,  and  approaches 
image  1,  proving  that  this  surface  has  become  more  convex 
and  approached  the  cornea. 

The  precise  mechanism  by  which  this  change  in  the  lens  is 
brought  about  is  not  so  easy  of  demonstration.  Two  com- 
monly-held theories  divide  the  field.  Both  of  these  theories 
agree  that  the  active  factor  in  the  process  is  the  contraction 
of  Mueller's  muscle,  the  sphinctre  muscle  of  the  ciliary;  but 
they  disagree  as  to  the  mechanical  efifect  of  its  contraction. 

Helmholz,  who  was  the  first  to  investigate  the  phenomenon 
of  accommodation,  taught  that  the  contraction  of  the  ciliary 
muscle,  by  drawing  forward  the  chorioid,  relaxes  the  tension 
of  the  zonula,  and  permits  the  lens,  in  virtue  of  its  elasticity, 
passively  to  assume  a  form  more  nearly  approximating  the 
spherical.  At  the  same  time  there  is  necessarily  produced  a 
corresponding  decrease  in  the  equatorial  diameter  of  the  lens ; 
its  equator  accordingly  recedes  inward  toward  the  axis  of  the 
eye,  and  is  thus  kept  from  coming  in  contact  with  the  ciliary 
processes  as  they  advance. 

According  to  Tscherning,  the  contraction  of  the  ciliary 
muscle  tightens  instead  of  relaxing  the  zonula,  altering  the 
lens  surface  from  a  spherical  to  a  hyperboloid  form.  zA.ccord- 
ing  to  this  theory  the  bulging  of  the  lens  into  more  convex 
shape  is  produced  by  an  active  compression  by  Mueller's  ring, 
and  not  by  a  passive  exercise  of  the  elasticity  of  the  lens. 

Both  theories  have  their  merits  and  demerits.  In  favor  of 
Helmholz's  theory  is  the  significant  fact  that  whenever  the 
lens  is  divided  from  the  fibres  of  the  zonula,  (as  in  extraction), 
it  rapidly  assumes  a  spherical  form.  In  support  of  Tschern- 
ing's  doctrine  is  the  equally  significant  fact  (which,  indeed, 
led  Tscherning  to  his  belief),  that  in  accommodation,  while 
the  central  part  of  the  anterior  surface  of  the  lens  is  bulged, 
the  peripheral  portion  is  flattened.  Helmholz's  theory  is  the 
more  generally  accepted,  although  it  does  not  satisfactorily 
account  for  all  the  phenomena  of  accommodation. 

The  actual  physical  change  efifected  in  the  crystalline  lens 
by  accommodative  muscular  effort  in  a  young  person  is  ex- 
pressed in  the  following  comparison  of  thickness  and  curva- 


10  ACCOMMODATION 

tures  in  the  static  condition  and  at  the  height  of  accommoda- 
tion : 

Far  Point.       Xear  Point. 

Thickness   Z.7  mm.  4  mm. 

Anterior  radius   10     mm.  6  mm. 

Posterior  radius    6     mm.  5  mm. 

This  comparison  shows  how  much  more  markedly  the  an- 
terior surface  is  affected  than  the  posterior. 

The  sphinctre  iridis  and,  if  binocular  vision  is  present,  the 
internal  rectus  muscles,  contract  in  association  with  the  ciliary 
muscle.  The  act  of  accommodation,  therefore,  is  regularly 
accompanied  by  a  contraction  of  the  pupil  and  by  the  act  of 
convergence. 

RELATIVE  ACCOMMODATION. 

Not  only  is  the  act  of  accommodation  regularly  accompanied 
by  the  act  of  convergence,  but  a  certain  quantitati^•e  ratio  is 
normal  between  the  two.  Thus,  for  1  meter  angle  of  con- 
vergence (i.  e..  con\ergence  to  1  meter  distance)  the  eye 
normally  exerts  1  D.  of  accommodation.  However,  it  is  always 
possible,  by  means  of  minus  lenses,  to  force  a  certain  amount 
of  additional  accommodation  at  this  point;  and  l)y  means  of 
convex  lenses  to  suppress  a  certain  amount.  These  forced 
and  suppressed  quantities  are  known,  respective!}-,  as  j)ositivc 
and  negative  relative  accommodation.  The  difference  between 
the  two  is  said  to  be  the  relatix  e  accommodation  of  the  eye. 

It  is  due  to  this  latitude  of  relative  accommodation  that 
every  ametrope  is  not  doomed  to  strabismus  and  amblyopia. 
For  if  the  ratio  between  accommodation  and  convergence  were 
an  al^solutely  fixed  quantity,  every  ametrope  would  have  to 
choose  between  two  alternatives:  either  to  see  distinctly  but 
double,  because  c»f  defective  convergence,  or  else  to  see  singly 
but  indistinctly,  because  of  the  defective  accomnuxlation. 

This  relative  amplitude  of  accommodation,  however,  has  its 
limits.  A  hyi^erope  of  a  certain  degree  may  succeed,  with  an 
effort,  in  achieving  the  excess  accommodation  demanded  by 
his  error  of  refraction  while  he  maintains  emmetropic  con- 
vergence. But  if,  for  any  reason, — such  as  nervous  disturb- 
ance, headache,  etc..  or  ine(|uality  of  visual  acuity  in  the  two 
retinae — it    should    be    much    easier    to   disregard    one    of    the 


ACCOMMODATION  11 

images  than  to  maintain  relative  accommodation,  then  he  is 
likely  to  lapse  into  squint.  Or,  if  the  amount  of  refractive 
error  be  greater  than  can  be  compensated  by  relative  accom- 
modation, then  the  patient  usually  chooses  the  alternative  last 
mentioned  above — to  see  singly  but  indistinctly;  or,  rather,  he 
gives  up  trying  to  see  closely  at  all,  just  as  the  myope  does. 

It  has  also  been  established  by  repeated  experiment  that  the 
amount  of  convergence  exerted  in  association  with  accommo- 
dation for  a  given  point  is  not  all  of  it  a  co-ordinate  of  the 
accommodation.  It  would  appear,  from  these  experiments, 
that  in  the  majority  of  cases  only  two-thirds  of  it  is  co-ordinate 
with  the  accommodation,  the  other  one-third  being  exercised 
separately  and  distinctly.  This  will  be  fully  discussed  in 
treating  of  convergence. 

The  exploration  of  a  patient's  relative  convergence  is  of  im- 
portance, because  it  represents  what  is  known  in  engineering 
as  the  "flexibility"  of  the  function.  It  measures,  in  fact,  the 
power  of  the  accommodation  center  to  function  independently 
of  its  association  with  convergence.  It  constitutes,  therefore, 
one  of  the  chief  factors  in  reaching  a  conclusion  as  to  the 
patient's  need  of  assistance. 

ACCOMMODATIVE  RESERVE. 

It  is  an  established  fact  that  no  person  can  exert  his  total 
amplitude  of  accommodation  for  any  great  length  of  time,  or 
for  ordinary  purposes.  It  is  usually  held  that  he  must  keep 
at  least  one-third  of  it  in  reserve.  This  estimate,  however,  as 
we  shall  see,  is  subject  to  considerable  variation  in  individual 
cases. 

MATHEMATICS  OF  ACCOMMODATION. 

The  optical  mathematics  of  accommodation  is  very  simple, 
although  it  may  give  rise  to  many  complicated  problems.  The 
normal  eye  in  its  static  condition  is  adapted  to  infinity,  i.  e., 
to  focus  neutral  light  waves  at  its  retinal  plane.  In  order  to 
focus  finite,  or  positive,  waves  at  the  same  focal  plane,  it  must 
necessarily  add  to  its  dioptrism  exactly  enough  power  to 
neutralize  the  positive  wave  and  render  it  as  though  it  came 
from  infinity.  In  other  words,  it  must  add  to  itself  the  exact 
dioptric  equivalent  of  the  positive  curvature  of  the  wave  at 


12  ACCOMMODATION 

the  point  where  it  strikes  the  cornea.  (It  is  to  be  observed, 
in  passing,  that  the  actual  physical  curvature  to  be  added  to 
the  crystalline  lens  must  be  greater  than  the  metric  curve  of 
the  light  wave  it  is  to  neutralize,  because  only  a  portion  of  the 
lens-curve  is  available  for  dioptric  effect.) 

Again,  the  distance  of  the  light  wave's  point  of  origin  from 
the  eye  is  the  radius  of  its  curvature  when  it  strikes  the  eye ; 
and  the  same  distance  is  the  radius,  or  focal  length,  of  the 
dioptric  power  which  the  eye,  by  its  accommodation,  adds  to 
itself  for  the  purpose  of  neutralizing  the  wave.  Whence  we 
deduce  the  simple  rule  that,  in  the  normal  eye,  accommoda- 
tion is  the  reciprocal  of  the  distance  for  which  it  is  exercised. 
Either  quantity  divided  into  unity  will  give  us  the  other.  Thus, 
for  an  object  at  50  cm.  the  eye  uses  2  D.  of  accommodation; 
if  it  exercises  4  D.  of  accommodation,  it  is  adapted  for  an  object 
25  cm.  distant. 

In  the  case  of  a  hyperope,  who  is  obliged  to  use  at  infinity 
an  amount  of  accommodation  equal  to  his  hyperopia,  this 
amount  must  be  added  to  the  reciprocal  of  the  distance.  Thus, 
if  a  person  with  2  D.  of  hyperopia  fixes  for  an  object  at  50  cm.. 
he  uses  2  D.  (the  reciprocal  of  .50)  plus  2  D.  (the  amount  of 
his  hyperopia)  of  accommodation.  In  the  case  of  a  myope, 
whose  far  point  is  within  infinity,  the  amount  of  his  myopia 
must  be  deducted  from  the  reciprocal  of  the  distance.  Thus 
if  a  myope  of  2  D.  fixes  for  an  object  at  25  cm.,  he  uses  4  D. 
(the  reciprocal  of  .25)  minus  2  D.  (the  amount  of  his  myopia) 
of  accommodation. 

The  preceding  paragraph  states  the  case  theoretically.  As  a 
matter  of  fact,  experience  has  shown  that,  even  when  allow- 
ance has  been  made  for  the  excess  or  deficit  due  to  refractive 
error,  the  amount  of  accommodation  exercised  by  a  hyperope 
for  a  given  near  point  is  always  somewhat  in  excess  of  normal, 
and  that  exercised  by  a  myope  somewhat  short  of  normal. 
The  probable  reason  for  this  will  Ijc  found  discussed  else- 
where. 

MEASURING  AMPLITUDE  OF  ACCOMMODATION. 

It  will  be  seen  that,  for  the  practical  purposes  of  optometry, 
it  is  highly  important  to  be  able  to  measure  the  amplitude  of 

acc()mni(jdatic)n,  for  with   this  information  in  our   possession 


ACCOMMODATION  13 

we  are  able  to  make  valuable  deductions  both  as  to  the  state 
of  the  patient's  refraction,  and  also  as  to  the  efficiency  or 
otherwise  of  his  accommodative  powers. 

The  very  simple  and  obvious  mathematical  premise  which 
lies  at  the  basis  of  all  our  calculations  is  that  the  amplitude  of 
accommodation  represents  the  difference  between  the  total 
dioptric  power  of  the  eye  and  its  static  refraction,  as  expressed 
in  the  formula 

a  =  p  —  r 
in  which  a  stands  for  amplitude,  p  for  total  dioptrism,  and  r 
for  static  refraction. 

Proceeding  to  the  practical  problem  of  determining  the  value 
of  r,  we  note  that  the  static  refraction  of  the  eye  is  the 
reciprocal  of  the  far  point,  or  punctum  remotum,  represented 
by  the  letter  R,  as  expressed  in  the  formula 

1 

R 

and  that  the  total  dioptrism  of  the  eye  is  the  reciprocal  of  the 
near  point,  or  punctum  proximum,  represented  by  the  letter 
P,  as  expressed  in  the  formula 

1 
P  =  — 

P 
Thus,  if  the  patient's  near  point  prove  to  be  10  cm.,  then  his 
total  developed  dioptrism  is  100/10  or  10  D.  If  his  far  point 
be  infinity,  then  his  static  refraction  is  100/inf.,  or  O  D.  Then, 
according  to  the  formula,  10  D.  —  O  D.  =  10  D.,  which  repre- 
sents his  amplitude  of  accommodation.  If  his  near  point  be 
10  cm,  and  his  far  point  50  cm.,  then  his  total  will  be  100/10, 
or  10  D.,  his  static  100/50,  or  2  D.,  and  his  amplitude  10  D.  — 
2  D.  =  8  D. 

DETERMINING  THE  FAR  POINT. 

Inverting  the  second  of  the  above  formulae  so  as  to  make 
R  the  quantity  sought,  we  have 

1 
R  =  — 
r 
but  as  r,  representing  the  static  refraction  of  the  eye,  is  in 
practice  usually  the  very  thing  that  we  are  ultimately  seeking 


14  ACCOMMODATION 

to  determine,  this  presentation  of  the  problem  has  not  much 
working  value.  We  must  look  for  other,  more  constructive 
methods  of  determining  R. 

Subjectively  we  can  ascertain  the  patient's  far  point  with 
some  degree  of  near-accuracy  by  means  of  his  ability  to  read 
at  a  given  distance  (usually  at  6  meters,  which  is  optical 
infinity)  test  letters  so  constructed  as  to  subtend  on  the  retina 
the  minimum  visual  angle  at  that  distance,  provided  the  dis- 
tance in  question  represents  his  far  point.  If  it  be  less  than 
his  far  point,  the  letters  will  subtend  a  greater  than  the  mini- 
mum visual  angle;  if  it  be  further  than  his  far  point,  it  will 
subtend  less  than  the  minimum  angle.  Hence,  the  smallest 
sized  type  he  is  able  to  read  at  the  given  distance,  or,  what  is 
the  same  thing,  the  distance  at  which  he  is  just  able  to  rea.d  a 
given  size  of  type,  furnishes  us  an  approximation  of  his  far 
point.  The  principle  of  these  letters  (known  as  Snellen's 
letters)  and  the  method  of  using  them,  will  be  found  fully 
discussed  elsewhere. 

A  much  more  accurate  method  of  determining  the  far  point 
is  the  objective  method,  by  means  of  the  retinoscope.  For  de- 
tails of  this  instrument  and  its  use  the  reader  is  referred  to  the 
section  on  Retinoscopy.  It  will  here  be  assumed  that  he  is 
familiar  with  the  technique.  If,  then,  we  place  a  strong  plus 
lens  before  the  patient's  eye,  and  find  the  point  of  reversal, 
that  point  will  represent  the  far  point  of  the  patient's  eye  i)Ius 
the  lens.  We  have  only  to  calculate  and  throw  out,  in  terms 
of  its  reciprocal,  the  distance  represented  by  the  lens,  and  the 
remainder  represents,  also  in  terms  of  its  reciprocal,  the  far 
point  of  the  eye  alone.  Reducing  it  to  a  formula,  if  f  stands 
for  the  focal  length  of  the  lens,  f  for  the  point  of  reversal, 
and  R  for  the  far  point,  then 

1  1  1 


f  f        R 

Thus,  if  we  place  a  i)lus  4  I ).  Ims  before  the  eye,  and  liml 
tin-  i)oint  of  rc'xersal  at  25  cm.,  we  haxc 

1  1  1 

2.S       25        0 


ACCOMMODATION  15 

showing  that  the  far  point  is  at  infinity.    Or  if,  with  the  same 
lens,  we  find  the  point  of  reversal  at  10  cm.,  then 
1  1  1 


10       25       16.6 
showing  that  the  far  point  is  at  16.6  cm. — he  is  a  myope  of  6  D. 
Or,  again,  if,  with  the  same  lens,  we  find  the  point  of  reversal 
at  50  cm.,  then 

1  1  1 

50       25       —50 
indicating  that  the   far   point  is   50  cm.   beyond   infinity — the 
patient  is  a  hyperope  of  2  D. 

It  is  true  that  both  these  methods  are  subject  to  sources  of 
error ;  but  they  are  the  sources  of  error  which  pertain  to  the 
test-chart  and  retinoscope  in  general,  and  therefore  are  to  be 
avoided  and  minimized  by  whatever  modifications  of  technique 
render  these  procedures  more  reliable  in  their  general  out- 
w'orking. 

DETERMINING  THE  NEAR  POINT. 

Theoretically,  the  determination  of  the  near  point  should  be 
a  simpler  matter  than  that  of  the  far  point,  since  (except  in 
presbyopes  whose  accommodation  is  lost)  the  near  point  is 
always  an  actual,  discoverable  point  within  infinity.  In  prac- 
tice, how'ever,  there  are  many  obstacles  in  the  way  of  an 
accurate  ascertainment  of  the  near  point,  even  more  insur- 
mountable than  those  which  pertain  to  the  far  point. 

For  one  thing,  the  near  point  being  a  function  of  active 
muscular  eft'ort,  is  hardly  ever  the  same  in  the  same  patient 
at  any  two  successive  examinations,  depending  upon  condi- 
tions of  general  health  fatigue,  mental  and  physical  concen- 
tration, etc.  And,  for  another,  the  exercise  of  accommodation 
is  very  largely  a  matter  of  habit,  so  that  it  is  exceedingly  diffi- 
cult to  get  the  patient  to  put  into  effect  more  accommodation 
than  he  is  in  the  habit  of  doing. 

Of  the  subjective  methods,  the  old  classical  one  is  the  pin- 
hole test  of  Scheiner.  Close  in  front  of  the  eye  to  be  tested  we 
place  a  card  pierced  with  two  tiny  holes,  which  must  not  be 
further  apart  than  the  diameter  of  the  pupil.     Through  these 


16  ACCOMMODATION 

holes  the  patient  views  a  very  small  object  (a  pin-head)  which 
is  gradually  brought  nearer  to  the  eye.  As  long  as  accommo- 
dation holds  out,  the  pin  will  be  seen  singly ;  but  as  soon  as 
the  pin  gets  nearer  to  the  eye  than  the  near-point  it  will  be 
seen  double. 

More  commonly  used  in  optometrical  practice  is  the  test- 
type  method,  very  similar  to  the  test  for  the  far  point,  except 
that  in  this  case  we  have  to  find  the  nearest  distance  at  which 
the  patient  can  read  a  given  type.  For  this  purpose  we  employ 
the  Jaeger  type,  or  some  modification  of  it,  which  is  con- 
structed substantially  on  the  same  visual  angle  principle  as 
Snellen's  distance  type.  However,  it  is  much  more  difficult 
to  approximate  accuracy  in  the  case  of  the  near  type,  for  the 
reason  that  at  such  a  short  distance  from  the  nodal  point  of 
the  eye  very  slight  changes  of  distance  cause  considerable 
variations  of  the  visual  angle,  and  it  is  almost  impossible  to 
devise  a  scale  of  letters  which  will  compensate  for  this  varia- 
tion. 

If  we  elect  this  method,  we  must  carry  it  out  as  carefully  as 
we  can,  and  avoid  sources  of  error  as  much  as  possible. 

Moreover,  with  the  exercise  of  accommodation  there  is  a 
corresponding  exercise  of  the  sphinctre  iridis,  causing  a  pro- 
gressive contraction  of  the  pupil ;  and  this,  acting  as  a  cut-ofF 
to  the  peripheral  rays  of  light,  has  a  distinctly  modifying  effect 
upon  the  clearness  of  the  image,  quite  apart  from  the  clear- 
ness of  the  focus.  However,  as  this  and  the  foregoing  "source 
of  error"  are  both  of  them  normal  to  the  functioning  of  accom- 
modation, their  vitiating  effect  ui)on  the  test  is  probably  aca- 
demic rather  than  practical.  After  all,  the  reading  of  small 
type  at  near  distance  is  the  normal  purpose  of  accommoda- 
tion ;  hence  this  test  will  probably  remain  the  commonest 
method  of  determining  the  near  point. 

Another  subjecti\e  method  is  to  find  a  ctmcave  lens  which 
will  serve  as  a  substitute  for  the  accommodation,  and  make 
light-waves  from  infinity  as  though  they  came  from  the  near 
point.  The  techni(|ue  (A  this  method  is  to  find  first  the  type  on 
the  Snellen  chart  which  represents  the  patient's  far  point,  and 
then  ])Ut  before  tlu-  patient's  eye  successively  stronger  minus 
lenses  until  he  can  no  longer  ri-ad  this  t\pe.     The  reciprocal 


ACCOMMODATION  17 

of  the  strongest  minus  lens  with  which  he  can  read  represents 
his  near  point. 

The  objection  to  this  method  is  precisely  the  opposite  of  the 
objection  to  the  near-type  test,  namely,  that  under  the  influence 
of  the  minus  lenses  the  visual  angle  becomes  so  small  that  the 
size  of  the  image  made  upon  the  retina  prevents  the  patient 
from  reading  the  distant  type  long  before  his  accommodation 
has  been  equalized.    It  is  a  poor  method  in  practice. 

For  the  near  point,  as  for  the  far  point,  the  objective  method 
of  determination  is,  on  the  whole,  the  most  satisfactory  when 
the  amplitude  of  accommodation  is  in  question.  The  carrying 
out  of  this  procedure  belongs  to  the  technique  of  dynamic 
refraction,  which  is  described  in  detail  elsewhere. 

Sheard  holds  that  the  near  point  cannot  be  accurately  de- 
termined by  dynamic  skiametry  as  it  is  generally  practiced. 
For  this  purpose  he  uses  the  following  technique :  With  full 
distance  correction  on,  the  patient  is  directed  to  fix  his  vision 
upon  a  letter  chart  held  two  or  three  inches  in  front  of  the 
mirror,  and  the  operator  notes  at  some  convenient  point  the 
movement  of  the  shadow.  If  it  is  "with"  he  pushes  the  chart 
forward  until  it  becomes  "against."  He  then  comes  forward 
with  the  mirror  until  it  is  "with"  again ;  and  so  on  until  with 
the  chart  some  distance  in  front  of  the  mirror  he  locates  the 
nearest  point  of  neutral  shadow,  which  denotes  the  patient's 
true  near  point. 

CALCULATING  THE  AMPLITUDE. 

Having  now  determined  both  the  far  point  and  the  near 
point,  we  divide  each  quantity  into  unity  to  find,  respectively, 
the  static  refraction  and  the  total  developed  dioptrism  of  the 
eye.  If  the  far  point  has  been  shown  to  be  within  optical 
infinity,  it  is  written  as  a  negative  quantity,  and  thus  divided 
into  unity ;  if  beyond  infinity,  it  is  written  as  a  positive 
quantity,  and  thus  divided.  Thus,  if  the  far  point  was  found 
to  be  at  4  meters,  the  static  refraction  is  — 1/4,  or  .25  D.  of 
myopia.  If  at  50  cm.  beyond  infinity,  then  it  is  1/50,  or  2  D.  of 
hyperopia. 

To  calculate  the  amplitude  of  accommodation,  we  then 
proceed  to  give  efifect  to  our  original  formula-. 

a  ^  p  —  r 


18  ACCOMMODATION 

l.ct  us  assume  that  in  hoth  the  illustrations  just  cited  the 
near  point  had  been  shown  to  be  at  10  cm.,  making  p  equal  to 
10  D.     Then,  in  the  first  example, 

a  =  10  —  (—.25)  =  10.25  D. 
and  in  the  second  instance, 

a  =  10  — 2  =  8D. 
and  in  a  case  where  the  far  point  was  demonstrated  at  infinity 
(emmetropia)  and  the  near  point  at  10  cm., 
a  =  10  —  inf.  =  10  D. 

MEASURING   RELATIVE   ACCOMMODATION. 

The  importance  of  ascertaining  the  amount  of  the  patient's 
relative  accommodation,  positive  and  negative,  has  already 
been  pointed  out.  In  conjunction  with  the  amount  of  relative 
convergence  (adduction  and  abduction)  it  represents  the'  flex- 
ibility of  these  two  functions.  The  nearer  this  relative  accom- 
modation approximates  the  normal,  the  more  we  can  rely  upon 
the  patient  being  able  to  maintain  proper  co-ordination  be- 
tween the  two  and  to  find  his  own  comfortable  near  point  when 
rendered  emmetropic  by  distance  correction. 

Positive  relative  accommodation  is  found  by  gradually 
adding  minus  lens  power  to  the  eye  while  convergence  is 
maintained  for  a  near  ])oint.  Negative  accommodation  by 
gradually  adding  plus  lens  power  under  like  conditions.  When 
vision  blurs,  in  each  case,  the  limit  is  reached.  Sheard  holds 
that  dynamic  skiametry,  as  generally  practiced,  is  but  a  method 
of  determining  negative  relative  accommodation. 

According  to  Bonders,  "The  accommodation  can  be  main- 
tained only  for  a  distance  at  which,  in  reference  to  the  nega- 
tive part,  the  ])ositive  ])art  of  relatixc  accommodation  is  tol- 
erably large.''  If,  therefore,  the  result  of  the  test  docs  not 
fulfill  this  condition — i.  e.,  if  the  positiv  c  relative  accommoda- 
tion is  not  noticeably  greater  than  the  negative — we  arc  jus- 
tified in  assuming  that  there  is  lack  of  tlexibility,  ami  that  the 
eyes  camiot  long  maintain  accommodation  for  the  ])oint  in 
question.  The  best  distance  at  which  to  make  the  test  is  the 
distance  at  which  the  patient  ordinarily  does  his  or  her  ne;*r 
work. 


ACCOMMODATION  19 

MEASURING   ACCOMMODATIVE  RESERVE. 

The  measurement  of  the  negative  relative  convergence  is, 
in  effect,  the  measurement  of  the  accommodative  reserve;  or, 
to  be  accurate,  it  furnishes  the  basis  for  its  calculation.  The 
closest  point  at  which  we  can  get  neutrality  of  shadow  is  the 
comfortable  near  point.  Converting  this  into  dioptres  (by- 
dividing  it  into  unity)  gives  us  the  maximum  amount  of  ac- 
commodation that  the  patient  can  use  without  eye-strain.  De- 
ducting this  from  the  total  amplitude  of  accommodation,  we 
obtain  the  reserve  accommodation  required  by  this  patient  at 
that  point. 

The  ratio  between  the  three  factors — the  total  amplitude, 
the  near  point,  and  the  required  reserve — indicates  the  flexi- 
bility of  the  accommodation.  Normally,  the  near  point,  in 
terms  of  dioptrism,  should  be  about  three  or  four  times  as 
much  as  the  required  reserve.  If  this  ratio  be  markedly  dis- 
turbed, accommodative  insufficiency  or  inefficiency  is  indicated, 

THE  COMFORTABLE  NEAR  POINT. 

It  is  formally  assumed  that  two-thirds  of  the  ampHtude  of 
accommodation  can  be  exercised  for  close  work  for  reason- 
ably long  periods  of  time  without  inconvenience  or  discomfort; 
in  other  words,  that  it  is  always  sufficient  to  have  one-third 
in  reserve  at  the  near-point.  Accommodation  being  a  dynamic, 
muscular  function,  however,  individuals  differ  in  this  respect. 
There  is,  undoubtedly,  a  comfortable  near  point  for  each  in- 
dividual, which  should  be  sought  for  and  determined  in  every 
case. 

One  method  of  finding  this  comfortable  reading  point  is  by 
dynamic  retinoscopy.  With  the  distance  correction  in  place, 
we  begin  to  shadow  dynamically  at  1  meter,  or  40  inches.  If 
the  movement  of  the  shadow  is  not  with  the  mirror,  we  take 
the  mirror  in  nearer  to  the  patient's  eye  until  it  is  seen  moving 
with.  We  then  back  oft'  until  the  shadow  is  abolished.  This 
point  represents  the  near  point  of  comfortable  vision,  for  this 
is  the  point  at  which  the  eye  naturally  harmonizes  its  accom- 
modation and  its  convergence. 

Another  method  is  the  cross-cylinder  of  R.  M.  Lockwood. 
After  the  refractive  error  has  been  corrected,  and  the  near 
vision  checked  by  appropriate  near  type  which   the   patient 


20  ACCOMMODATION 

reads  clearly,  a  compound  lens  equivalent  to  a  cross-cylinder, 
say  a  plus  .50  sphere  and  a  minus  1  cylinder  (which  is  equiva- 
lent to  cross-cylinders  of  .50  each)  is  put  before  the  eye,  creat- 
ing a  false  astigmatism  of  .50  hyperopic  in  one  and  .50  myopic 
in  the  other.  The  type  will  blur,  but  if  for  the  type  there  is 
now  substituted  a  T  chart,  with  the  arms  of  the  T  made  to 
coincide  with  the  axes  of  the  false  astigmatism,  then,  if  before 
applying  the  test  the  eyes  were  in  exact  focus  there  will  be 
no  perceptible  difTerence  in  the  two  arms  of  the  T;  but  if  the 
focus  was  not  exact,  then  the  test  will  show  a  difference,  and 
the  spherical  part  of  the  correction  must  be  altered  until  this 
difference  is  not  perceptible. 

The  efficiency  of  the  cross-cylinder  test  can  be  increased 
by  having  a  set  of,  say,  three  T  charts  of  different  sizes.  The 
cross  cylinders  being  applied,  as  described  above,  the  srhallest 
T  chart  should  be  brought  close  to  the  eye  and  slowly  with- 
drawn until  both  lines  become  equally  clear.  If  this  does  not 
occur  by  the  time  a  distance  say  of  12  inches  is  reached,  sub- 
stitute the  second  T  chart  out  to  about  24  inches,  and  then 
the  larger  chart  to  about  40  inches  until  the  point  of  equaliza- 
tion is  found.  Converted  into  dioptres,  this  gives  the  com- 
fortable amplitude  of  accommodation. 

SUMMARY. 
Recapitulating,  the  investigation  of  the  patient's  accommo- 
dative status  involve?  the  following  practical  steps: 

1.  Ascertain  the  patient's  far  point.  This  is  equivalent  to 
determining  the  static  refraction  of  the  eyes,  and  is  to  be  done 
by  the  usual  methods,  viz.,  Snellen's  test  types,  subjectively, 
and  static  retinoscopy,  objectively.  The  far  point  may  be 
found  to  differ  in  different  meridians  of  the  eye  (astigma- 
tism). 

Having  found  the  far  point,  if  it  denotes  an  error  of  static 
refraction,  it  is  best  to  correct  this  error,  thus  making  the 
patient  emmetropic,  with  a  far  i)C)int  at  infinity,  before  pro- 
ceeding to  further  tests. 

2.  Ascertain  the  near  point.  This  is  to  be  done  in  various 
ways, — by  Scheiner's  two-hole  test,  by  Jaeger  test  types,  by 
minus  lens  power  with  which  the  patient  can  read  20/20,  or 


ACCOMMODATION  21 

by  dynamic  skiametry,  Sheard's  method.     Of  these,  the  last 
named  is  undoubtedly  the  most  accurate. 

Having  found  the  near  point,  subtract  the  far  point  from 
it,  in  terms  of  dioptrism,  to  find  the  amplitude  of  accommo- 
dation. 

3.  Make  a  still  finer  test  of  the  patient's  accommodative 
efficiency  by  means  of  Lockwood's  cross-cylinder  method. 

4.  Test  out  the  patient's  relative  accommodation  by  the 
addition  of  minus  and  plus  lenses,  respectively,  at  near  point. 
Especially  explore  the  negative  relative  accommodation,  for 
the  purpose  of  determining  the  amount  of  reserve  necessary 
in  his  case. 

5.  On  the  comparative  results  of  all  these  data  decide  as 
to  the  desirability,  or  otherwise,  of  assisting  the  patient's  near 
vision,  without  regard  to  any  stereotyped  rules  of  practice. 

ANOMALIES  OF  ACCOMMODATION. 

Within  reasonable  limits  the  above  tests  will  give  us  ap- 
proximately the  available  amplitude  of  accommodation,  or,  at 
least,  the  amplitude  which  the  patient  activates.  This  will 
coincide  with  the  real  total  amplitude  only  when  there  exist  no 
modifying  conditions  which  prevent  the  free  interplay  of  the 
various  elements  in  the  equation.  If  the  amplitude,  as  de- 
termined by  the  tests,  measures  up  to  what  is  to  be  expected 
at  the  patient's  age,  we  may  assume  that  no  such  vitiating 
conditions  exist.  If  not,  we  must  assume,  on  the  contrary, 
the  presence  of  some  anomaly  of  accommodation,  and  apply 
further  tests  in  a  search  for  the  trouble. 

CILIARY    SPASM. 

Commonest  among  these  accommodative  anomalies  is  a 
periodic  (clonic)  or  a  permanent  (tonic)  contraction  of  the 
ciliary  muscle,  which  frequently  occurs  in  hyperopia,  and  which 
results  in  a  certain  amount  of  locked  accommodation  that  is 
not  easy  to  demonstrate  by  any  of  the  methods  described 
above. 

Subjectively,  such  a  patient's  far  point  would  register  inside 
of  its  mathematical  range,  because  he  is  not  able  to  attain 
complete  ciliary  relaxation.  It  may  register  at  infinity,  in 
which  case  he  will  appear,  so  far  as  his  far  point  is  concerned, 


22  ACCOMMODATION 

to  be  emmetropic;  or  it  may  even  be  within  infinity,  so  that  a 
hasty  conclusion  would  set  him  down  as  a  myope. 

The  capital  symptom  of  all  anomalies  of  accommodation, 
naturally,  is  a  discrepancy  in  the  relative  positions  of  far  point 
and  near  point.  The  nature  of  the  discrepancy  is  a  guide  to 
the  nature  of  the  anomaly.  In  hyperopic  spasm  of  the  ciliary, 
the  far  point  is,  as  just  explained,  either  at  infinity  or  even 
within  infinity,  while  the  near  point  is  considerably  further  out 
than  normal.  This,  coupled  with  symptoms  of  accommodative 
asthenopia,  leads  us  to  suspect  the  presence  of  a  ciliary  spasm. 

The  proof  of  the  existence  of  the  spasm  lies  in  inducing  the 
patient  to  surrender  all  or  part  of  it,  thus  demonstrating  it. 
There  are  several  ways  of  doing  this,  all  of  which  will  be  found 
fully  discussed  in  the  section  devoted  to  hyperopia.  Patients 
can  usually  be  made  to  relinquish  all  of  a  clonic  spasm,  and  a 
portion  of  a  tonic  spasm. 

ACCOMMODATIVE   INSUFFICIENCY. 

This  condition,  of  course,  is  normal  at  and  after  the  age  of 
45,  when  it  goes  by  the  name  of  presbyopia.  It  is  only  when 
it  is  found  in  younger  persons,  and  when  in  presbyopes  the  in- 
sufificiency  exceeds  the  amount  proper  to  their  age,  that  it  is 
regarded  as  an  anomaly,  and  is  known  as  accommodative  in- 
sufficiency, or  subnormal  accommodation. 

It  is  a  little  difficult  to  draw  a  hard  and  fast  line  between 
insufficiencies  due  to  actual  paralysis,  complete  or  partial 
(paresis)  of  the  ciliary  muscle,  and  those  that  are  functional. 
Some,  of  course,  belong  entirely  in  the  former  class,  and  are 
then  to  be  regarded,  not  as  cases  of  insufficiency,  but  of 
paralysis  or  paresis,  and  will  be  discussed  under  that  heading. 
Others  belong  just  as  definitely  in  the  purely  functional  class, 
where  no  lesion  exists,  but  the  trouble  is  in  a  faulty  innerva- 
tion of  the  muscle.  Still  others  constitute  a  sort  of  borderland 
group  (e.  g.,  neurasthenia)  in  which  it  is  hard  to  say  whether 
the  trouble  is  functional  or  toxic — perhaps  a  mixture  of  both 
In  a  large  proportion  of  cases  it  seems  impossible  to  assign 
any  definite  cause  at  all. 

The  prime  symptom  is  naturally  an  inability  to  see  at  a  con- 
venient near-point — premature  i)resbyopia,  or  (in  case  of  an 
older   i)ers(»n)    excessive   presbyopia.     The   near   point,   as   in 


ACCOMMODATION  23 

ciliary  spasm,  is  considerably  further  out  than  normal.  In- 
vestigation shows  a  contraction  of  the  range,  and  diminution 
of  amplitude,  of  accommodation.  The  tests  for  ciliary  spasm 
reveal  none.  Relative  accommodation  is  either  lacking  or  much 
reduced.  Attempts  to  force  accommodation  fail.  There  is, 
however,  no  trouble  until  the  patient's  near  point  is  reached. 

ACCOMMODATIVE  INEFFICIENCY. 

This  condition  is  to  be  carefully  distinguished  from  insuf- 
ficiency. Its  existence  and  importance  is  of  comparatively 
recent  recognition.  It  consists,  essentially,  not  in  any  lack  of 
muscular  or  nervous  power,  but  in  an  inability  to  use  them 
properly.  Many  persons,  for  some  reason  or  other,  never 
achieve  their  own  potential  near-point ;  others  never  attain  a 
comfortable  relative  co-ordination  between  accommodation 
and  convergence,  and  therefore  cannot  (or  do  not)  hold  a  near- 
point  long  when  they  find  it.  Apparently  no  definite  cause 
can  be  assigned.  It  seems  to  be  simply  a  faulty  habit  of  func- 
tioning. 

These  are  the  cases  which,  above  all  others,  call  for  thorough 
investigation  of  the  accommodation-convergence  relation,  posi- 
tive and  negative  relative  accommodation,  and  accommodative 
reserve,  and  whose  treatment  taxes  the  judgment  of  the  re- 
fractionist.  Needless  to  say  that  accurate  distance  correction, 
where  needed,  is  of  great  importance ;  although,  to  be  sure, 
many  of  these  cases  of  inefiiciency  are  found  in  emmetropes. 
Emmetropia  being  present,  or  being  brought  about  by  distance 
correction,  inefficient  accommodation  must  be  acknowledged 
when  a  patient  proves  to  have  sufficient  amplitude  but  cannot 
use  it  at  near  point  without  eyestrain. 

As  to  the  treatment  of  such  cases,  that  is  a  matter  of  ex- 
ceedingly nice  judgment  for  the  operator.  They  need  some 
kind  of  help.  No  doubt  the  best  kind  of  assistance,  if  prac- 
ticable, is  indirect  assistance,  which  will  induce  proper  co-ordi- 
nation between  accommodation  and  convergence.  But  if  neces- 
sary, the  refractionist  need  not  hesitate  to  give  reading  lenses. 
The  idea  that  reading  addition  must  not  be  given  to  young 
persons  belongs  to  a  by-gone  period. 

The  situation  in  regard  to  accommodative  inefficiency  is 
best  summed  up  in  Lockwood's  formulary : 


24  ACCOMMODATION 

1.  The  exact  functioning  of  accommodation  depends  upon 
a  perfectly  balanced  innervation  of  the  ciliary. 

2.  Emmetropia  is  merely  the  zero  of  ocular  refractive  meas- 
urements, and  has  no  relation  to  ciliary  innervation. 

3.  There  are  no  sure  methods  of  detecting  faulty  ciliary 
innervation  at  a  distance. 

4.  It  is  readily  detected  at  near  point  by  several  methods. 
(The  best  are  dynamic  skiametry  and  his  own  cross-cylinder 
test.) 

5.  Glasses,  to  be  comfortable,  must  be  such  as  to  produce 
balanced  ocular  innervation. 

Accommodation,  Line  Of.  The  distance  in  space  along  which 
there  is  clear  \ision  ot  an  object,  in  spite  of  the  disparity  of 
the  retinal  points.  This  distance  depends  upon  the  width  of 
the  pupil,  the  visual  acuity,  and  the  dioptrism  of  the  eye. 

Accommodometer.    See  Punctumeter. 

Achloropsia.  Inability  to  distinguish  the  color,  green.  A  tech- 
nical term  for  green-blindness. 

Achromatism.  Absence  of  chromatic  aberration.  (See  Aberra- 
tion.) Such  a  state  is  only  relative,  as  absolute  achromatism 
is  impossible  of  attainment. 

Achromatic  lenses  are  made  by  the  admixture  of  two  kinds, 
or  more,  of  glass,  ha\ing  different  optical  properties,  each  of 
which  counteracts  to  a  great  extent  the  chromatic  aberration 
of  the  others. 

Achromatopsia.    A  technical  term  for  color-blindness. 

Actinic.  In  the  white  beam  of  light,  mostly,  but  not  altogether, 
l)e\()ii(l  the  \iolet.  are  certain  waves  or  rays  which,  falling  on 
the  retina,  do  not  register  the  sensation  of  \ision,  but  exercise 
a  chemical  action  upon  the  structures  of  the  eye.  These  are 
called  actinic  rays.  'Jhc  })recisc  nature  of  their  action  is  un- 
known, but  it  is  understood  to  be  irritant,  and  it  is  now  well 
established  that  these  rays  play  a  primary  role  in  the  produc- 
tion of  cataract,  not  by  their  direct  action  upon  the  crystalline 
lens,  but,  being  dispersed  by  the  fibres  of  the  lens,  they  are 
reflected  and  refracted  against  the  ciliary  body,  whose  func- 
tion they  impair,  thus  depriving  the  lens  of  its  nulrinient. 


ACTINISM  25 

Many  attempts  have  been  made  to  construct  lenses  which 
will  absorb  the  actinic  rays  and  prevent  their  entrance  into 
the  eye.  As  they  are  mostly  in  the  upper  part  of  the  spectrum, 
such  lenses  are  mostly  made  to  keep  out  the  violet  and  ultra- 
violet rays,  and  are  therefore  usually  of  a  yellow  or  amber  color. 
The  Crookes  lens  is  a  conspicuous  and  successful  example  of 
this  type  of  lens. 

Actinism.    The  eflfects  of  actinic  rays  upon  the  eyes. 

Acuity.  In  optics  this  word  has  two  applications.  As  applied 
to  an  image  it  indicates  the  sharpness  and  clearness  with  which 
the  details  are  outlined.  As  applied  to  the  function  of  vision, 
it  signifies  the  keenness  with  which  the  retinal  image  is  per- 
ceived  (visual   acuity). 

The  acuteness  of  an  image  depends  upon  the  perfection  with 
which  it  is  focussed,  and  the  nice  balance  of  illumination.  (See 
Image.) 

Visual  acuity  is  a  physio-nervous  attribute,  made  up  of  two 
elements,  viz.,  the  peripheral  element,  i.  e.,  the  sensibility  of 
the  rods  and  cones  to  light  stimulus,  and  the  central  element, 
i.  e.,  the  reaction  of  the  brain  to  transmitted  impulses.  The 
latter  element  is  assumed  to  be  constant  in  all  normal  indi- 
viduals. The  test  of  visual  acuity,  therefore,  is  in  reality  a 
test  of  the  integral  state  of  the  retinal  rods  and  cones.  In 
order  for  two  points  of  light  to  be  perceived  as  two  separate 
points  (which,  of  course,  is  the  basis  of  the  perception  of  the 
details  of  an  image),  their  respective  foci  must  fall  on  separate 
rods  and  cones.  For  this,  again,  it  is  necessary  that  the  foci 
be  separated  by  an  angular  distance  of  at  least  1  minute  (mini- 
mum visual  angle).  A  test  of  visual  acuity,  therefore  (i.  e., 
of  ability  to  distinguish  details  of  a  letter,  picture,  etc.)  under 
the  minimum  visual  angle,  determines  with  the  greatest  degree 
of  accuracy  the  integrity  of  the  rods  and  cones.  If  there  be 
diseased  rods  and  cones  sprinkled  among  the  healthy  ones 
some  of  the  points  of  light  will  focus  on  these  and  vision  will 
be  indistinct. 

Two  common  errors  should  be  noted  here.  First,  despite 
our  assumption,  the  reaction  of  the  brain  to  an  image  is  not 
alike  in  all  normal  individuals.     Like  every  other  brain  reac- 


26  ADAPTATION  OF  THE  RETINA 

tion,  it  depends  largely  upon  training.  An  Indian  has  better 
visual  acuity  than  a  civilized  man,  not  because  he  has  a  better 
retina,  but  because  his  brain  is  trained  to  react  better  to  images. 
The  same  difference,  in  lesser  degrees,  obtains  between  indi- 
viduals of  the  same  race.  In  certain  unusual  conditions  it 
obtains  in  very  marked  degree.  (See  Amblyopia  Ex  Anopsia.) 
Second,  the  so-called  "visual  acuity  test"  which  every  rclrac- 
tionist  makes  and  records  as  a  preliminary  to  his  examination 
is  not  a  test  of  the  visual  acuity  at  all,  but  of  the  visual  angle. 
If  the  minimum  visual  angle  is  larger  than  it  ought  to  be  (as 
evidenced  by  the  V/v  fraction),  it  may  be  due  to  an  error  of 
refraction  or  to  a  genuine  lowered  visual  acuity.  Only  when 
the  inability  to  read  the  proper  test  type  has  been  shown  not 
to  be  due  to  an  error  of  refraction  does  such  a  test  furnish 
a  real  indication  and  measure  of  the  visual  acuity. 

Adaptation  of  the  Retina.  The  faculty  of  the  retina  of  adapting 
itself  to  variations  in  intensity  of  light.  Adaptation  from  dark 
to  light  is  called  "light  adaptation" ;  that  from  light  to  dark, 
"dark  adaptation."  Sudden,  marked  changes  require  an  ap- 
preciable time  for  such  adaptation  to  be  made,  as  everyone 
knows  by  common  experience.  Diseases  of  the  retina  and  those 
which  interfere  with  the  action  of  the  retina,  inhibit  this 
faculty. 

Adduction.  In  general,  this  term  signifies  the  turning  of  the 
eyes  inward  from  parallelism  toward  the  median  line  by  the 
internal  rectus  muscles — the  muscular  mechanism  of  con- 
vergence. No  doubt  the  obliques  assist  in  the  act,  but  they 
are  not  essential  to  it.  It  is  a  voluntary  act,  but  as  ordinarily 
performed  is  subconscious,  and  is  stimulated  byaccommodation 
and  the  fusion  center. 

Technically,  the  word  has  come  to  be  applied  to  the  artificial 
exercise  of  the  internal  recti,  without  the  associated  function 
of  accommodation,  by  means  of  prisms,  base  out,  by  placing 
stronger  and  stronger  prisms  before  the  eyes,  while  the  pa- 
tient tries  to  maintain  single  xision  of  a  small  object  at  infinity, 
the  internal  recti  arc  l)rought  into  play  to  the  limit  of  their 
ability,  while  no  accommodation  is  exercised.  Adduction  sel- 
dom develops  more  than  about  24  prism  dioptres  of  power  in 


ADVANCEMENT  27 

the  internals,  whereas  natural  amplitude  of  convergence  in  a 
normal  person  is  usually  more  than  30  prism  dioptres. 

Advancement.  An  operation  for  the  cure  of  squint,  in  which 
the  longer  muscle  or  tendon  of  the  squinting  eye  is  cut  and 
attached  to  the  eyeball  at  a  more  anterior  point. 

After-images.  The  effects  of  retinal  stimulation  by  light  last 
for  an  appreciable  time  after  the  stimulus  itself  has  ceased, 
giving  rise  to  what  are  known  as  after-images.  These  effects 
consist  in  (a)  a  continuation  of  the  primary  action  of  the 
stimulus,  in  accordance  with  the  principles  of  inertia,  pro- 
longing the  perception  of  the  original  image,  and  (b)  the  re- 
action of  fatigue,  due  to  exhaustion  of  the  stimulated  sub- 
stance in  the  retina,  giving  the  sensation  of  an  image  in  which 
the  relations  of  light  and  shade  and  color  are  inverted.  The 
former  result  is  known  as  a  positive,  and  the  latter  as  a  nega- 
tive, after-image. 

Thus,  we  see  a  rising  rocket  as  a  streamer  of  light,  because 
the  infinite  series  of  luminous  points  made  by  the  rocket  in 
its  movement  upward  make  upon  the  retina  a  corresponding 
series  of  stimulations  which  persist  after  the  rocket  has  passed 
these  points,  producing  an  infinite  linear  series  of  positive 
after-images.  It  is  for  the  same  reason  that  when  a  revolving 
wheel  or  top  reaches  a  certain  speed  it  no  longer  appears  to 
move;  the  revolving  spokes  or  segments  successively  stimulate 
rods  and  cones  of  the  retina  which  are  still  reacting  to  the 
positive  image  of  the  previous  spoke  or  segment,  so  that  the 
sensation  is  of  one  continuous  image. 

In  after-images,  it  is  often  possible  for  the  eye  to  perceive 
detail  which  escaped  notice  in  the  primary  stimulation  on  ac- 
count of  excessive  brightness.  Thus,  if  one  looks  at  a  tree 
standing  between  himself  and  bright  sun,  he  cannot  perceive 
the  small  branches  or  twigs ;  but  on  closing  the  eyes,  these 
become  visible,  because  as  the  image  fades  the  chiaroscuro 
relations  are  changed,  and  the  intensity  is  moderated. 

If  the  object  viewed  is  not  very  bright,  and  is  looked  at  for 
several  seconds,  the  positive  after-image  is  hardly  perceived, 
but  the  negative  after-image  immediately  presents  itself.  As 
stated  the  light  and  shade  relations  are  reversed,  as  in  the  nega- 


28  ALBINISM 

tive  of  a  photograph-film,  and  in  normal  cases  the  colors  in 
the  after-image  are  the  complementaries  of  those  in  the  original 
image.  It  is  noteworthy,  however,  that  red  images  may  give 
positive  after-images  which  are  complementary  in  color. 

The  apparent  size  of  an  after-image  depends  upon  the  dis- 
tance to  which  it  is  projected,  due,  of  course,  to  the  fact  that 
a  small  near  object  stimulates  the  same  area  of  the  retina  as 
a  larger  object  further  away,  and  vice  versa. 

Normally,  after-images  are  not  noticed,  in  accordance  with 
the  physiological  law  that  we  attend  only  to  those  things 
which  interest  us.  It  is  a  matter  for  the  judgment  of  the 
examiner,  how  far  after-images  and  entoptic  phenomena  are 
normal. 

Albinism.  A  condition  in  which,  in  consequence  of  some  biologic 
defect,  the  dark  coloring  matter  or  pigment  is  absent  from  the 
skin,  hair  and  eyes.  The  skin  is  milky,  the  hair  white,  the  iris 
pale  rose  color,  and  the  pupil  intensely  red  because  the  ab- 
sence of  pigment  allows  the  blood-vessels  to  be  clearly  seen. 
The  eyes  of  albinos  are  naturally  not  adapted  to  bright  sun- 
light, but  see  best  in  the  shade  or  in  the  moonlight.  There 
are  conditions  of  partial  albinism. 

Alexia.  The  word  denotes  inability  to  read,  by  which  is  meant 
not  illiteracy,  but  some  defect  in  the  reading  faculties.  It  may 
be  of  one  of  two  kinds:  (1)  inability  to  recognize  the  word- 
images,  due  to  a  lesion  between  the  occipital  and  frontal  lobes 
of  the  brain,  or  (2)  inability  to  associate  the  word-pictures, 
due  to  disease  of  the  frontal  areas  of  the  brain. 

Amacrine.     A  uni-polar  nerve-cell  found  in  the  retina. 

Amaurosis.  Absolute  blindness,  from  whatever  cause,  as  dis- 
tinct from  amblyopia,  which  denotes  relative  defect  of  vision. 
A  special  aiiplication  of  the  word  is  found  in  the  term 
Amaurotic  Family  Idiotcy,  a  rare  affection  of  the  retina  which 
occurs  in  young  children  during  the  first  two  years  of  life. 
The  area  of  the  macula  is  occupied  by  a  grayish-white  patch, 
about  as  large  as  the  oi)tic  disc,  with  a  red  spot  in  the  center. 
The  rest  of  the  fundus  is  usually  normal.  These  patients  gradu- 
ally become  blind,  and  within  a  few  nu»nths  die.     The  cause 


AMBLYOPIA  29 

of  the  disease  is  unknown.   It  attacks,  most  often,  the  children 
of  Jewish  parents. 

Amblyopia.  The  word  actually  means  blindness,  but  it  is  used 
to  signify  loss  of  vision,  partial  or  complete,  in  which  no  organic 
change  can  be  discerned  in  the  retina. 

The  commonest  form  of  amblyopia  is  that  due  to  disuse  of 
the  eye.  For  some  reason,  usually  to  avoid  diplopia  in 
strabismus,  the  patient  has  learned  to  disregard  the  image  in 
one  of  the  eyes,  and  the  brain  has  therefore  ceased  to  react  to 
the  image,  (Amblyopia  ex  anopsia).  By  persistent  and  prop- 
erly directed  exercises  of  the  eye  this  form  of  amblyopia  can 
often  be  overcome.  (See  Amblyscope.)  A  simple  measure 
of  cure  is  to  cover  the  sound  eye  for  long  periods  at  a  stretch, 
and  force  the  patient  to  use  the  amblyopic  eye. 

Another  form  of  the  disorder  is  toxic  amblyopia,  due  to  ex- 
cessive smoking,  drinking,  etc.,  in  which  the  rods  and  cones 
of  the  retina  become  temporarily  paralyzed.  Drugs,  such  as 
quinine,  also  occasionally  cause  it.  It  is  usually  central,  mani- 
festing itself  as  a  central  scotoma.  Abandonment  of  the  thing 
that  causes  it  will  often  result  in  restoration  of  vision. 

A  third  common  form  is  hysterical  amblyopia,  seen  chiefly 
in  women.  This  form  comes  on,  as  a  rule,  very  suddenly,  and, 
after  lasting  for  a  variable  length  of  time,  disappears  just  as 
suddenly. 

Amblyoscope.  An  apparatus  for  the  re-education  of  the  vision 
in  amblyopia  ex  anopsia.  The  Worth-Black  is  the  standard 
form   of   amblyoscope.      It    consists   essentially   of   two    eye- 


The  Worth-Black  Amblyoscope. 


30  AMETROPIA 

pieces,  similar  to  an  opera  glass,  which  can  be  turned  on  a 
swivel  to  adapt  themselves  to  any  degree  of  convergence.  At 
the  end  of  each  tube  is  mounted  a  different  picture  card,  which 
together  make  up  a  composite  scene,  (e.  g.,  a  bird  on  one  card 
and  a  bird-cage  on  the  other).  To  begin  with,  the  picture 
corresponding  to  the  amblyopic  eye  is  highly  illuminated,  the 
other  one  very  poorly,  so  as  to  draw  attention  to  the  image  on 
the  amblyopic  retina.  The  patient  tries  to  bring  the  two 
images  into  fusion  (e.  g.,  to  place  the  bird  in  the  cage),  thus 
exercising  both  covergence  and  attention  faculty  at  the  same 
time.  Gradually,  the  illumination  of  the  two  images  is  equal- 
ized, until  the  patient  can  see  both  images  equally  well  with 
equal  illumination. 

Ametropia.  A  general  term  applied  to  that  condition  of  the  eye 
in  which  there  is  an  error  of  refraction  of  any  kind. 

Amotio  Retinae.    Detachment  of  the  retina 

Amphiblestritis.    Another  term  for  retinitis. 

Amphodiplopia.     Diplopia  in  both  eyes. 

Amplitude  of  Accommodation.  The  total  amount  of  dioptric 
power  which  the  eye  is  capable  of  adding  to  itself  by  the  maxi- 
mum contraction  of  its  ciliary  muscle.     (See  Accommodation.) 

Amplitude  of  Convergence.  The  total  amount  of  deviation  in- 
ward from  the  median  line  which  the  eyes  can  make  by  maxi- 
mum contraction  of  the  internal  rectus  muscles.  (See  Con- 
vergence.) 

Anacamptics.     The  study  of  reflection  of  sountl  or  light. 

Anaphoria.  Tendency  of  the  eyes  to  turn  upward.  Sec  Hetero- 
phoria. 

Anatomy  of  the  Eye.  The  eye  is  a  hollow  spherical  globe,  made 
up  of  three  coats,  or  tunics,  hereafter  to  be  described,  l^ach 
eye  is  set  in  a  pyramidal  cavity  in  llu-  upper  frontal  part  of 
the  skull  made  by  the  union  of  seven  oi  the  cranial  bones, 
namely,  the  frontal  bone,  the  sphenoid,  the  etlunoiil.  the  su- 
perior maxillary,  the  malar,  the  lachrymal,  and   the  palatine. 


ANATOMY  OF  THE  EYE 


31 


Three  of  these,  the  frontal,  ethmoid,  and  sphenoid,  being  cen- 
tral portions  of  the  skull,  are  common  to  both  orbits ;  so  that, 
in  reality,  the  two  orbits  are  composed  of  eleven  bones  only. 


From  Hirschfeld's  Charts. 
a.  Superior  eyelid. — b.  Posterior  eyelid  showing  its  different  layers. — c,  c. 
Reflection  of  the  conjunctiva  on  the  posterior  face  of  the  eyelid,  and  on  the 
anterior  face  of  the  ocular  globe. — d.  d.  Orbito  ocular  aponeurosis,  prolonged 
en,  e,  the  sheath  of  the  optic  nerve,  and  on  the  sheaths  of  the  muscles. — f. 
Superior  rectus,  and  g,  Inferior  rectus. — h,  h.  Sclerotic  coat  thickened  pos- 
teriorly by  the  sheath  of  the  optic  nerve,  and  anteriorly  by  the  expansion  of 
the  aponeurosis  of  the  recti  muscles.— i.  Transparent  cornea  cut  to  show  its 
laminated  texture. — j,  j.  Choroid  coat. — k.  Ciliary  circle — I.  Ciliary  bodies  and 
processes. — m.  Iris  and  pupil. — n,  n.  Canal  of  Fontana. — o,  o.  Retina,  con- 
tinuous with  the  substance  of  the  optic  nerve. — p.  Ciliary  circle  of  Zinn. — 
q.  q.  Hyaloid  membrane. — r.  Capsular  artery  lodged  in  the  hyaloid  canal. — 
s,  s.  Vitreous  humour  and  Its  cells. — t.  Crystalline  lens  and  its  capsule. — 
u,  u.  Ruflled  canal,  or,  Canal  of  Petit. — v.  Anterior  chamber. — x.  Posterior 
chamber. 

The  bases,  or  large  openings,  of  the  orbits,  look  toward 
the  front;  the  apices  look  backward  and  slighly  inward.  At  the 
apex  of  each  orbit,  toward  the  nasal  side,  is  an  opening,  or 
foramen,  about  5  mm.  in  diameter,  known  as  the  optic  fora- 
men, through  which  the  optic  nerve  and  the  ophthalmic  artery 
emerge  from  the  cranium  to  enter  the  eye.  This  is  the  largest 
of  the  orbital  foramina,  of  which,  however,  there  are  eight 
others,  transmitting  nerves  and  blood-vessels  as  follows : 


32  ANATOMY  OF  THE  EYE 

Spheno-maxillary  fissure:  Superior  maxillary  nerve,  infra- 
orbital vessels,  and  ascending  branches  of  the  spheno-palatine 
or  Meckel's  ganglion. 

Sphenoidal  Fissure:  Third,  fourth,  three  branches  of  the 
ophthalmic  division  of  the  fifth,  and  sixth  cranial  nerves,  some 
filaments  of  the  sympathetic,  the  orbital  branch  of  the  middle 
meningeal  artery,  and  the  ophthalmic  vein. 

Supra-orbital  Fissure:  Supra-orbital  artery,  vein  and  nerve. 

Anterior  Ethmoidal  Foramen:  Anterior  ethmoidal  vessels 
and  nasal  nerve. 

Posterior  Ethmoidal  Foramen :   Posterior  ethmoidal  vessels. 

Infra-orbital  Fissure:    Infra-orbital  artery,  and  vein. 

Malar  Foramen :   Xerves  and  vessels  of  the  orbit. 

THE  EYEBALL. 
The  eyeball  itself,  as  stated,  is  made  up  of  three  coats,  or 
tunics,  one  outside  the  other,  each    having    substantially    a 
hollow  spherical  form,  as  follows: 

(1)  Inner,  or  nervous,  coat.  This  consists  of  a  flaring  of 
the  optic  nerve  into  a  layer  of  nervous  elements,  extending 
forward  about  two-thirds  of  the  eyeball,  and  forming  the  retina. 

(2)  Middle,  or  musculo-vascular  coat.  This  coat  is  com- 
prised of  muscle  tissue  and  vessels ;  it  starts  at  the  circumfer- 
ence of  the  optic  nerve  where  it  enters  the  eye,  extends  slightly 
more  forward  than  the  retina,  and  consists  of  three  portions, 
(a)  the  chorioid,  (b)   the  ciliary  body,  and   (c)  the  iris. 

(3)  Outer,  or  supporting  tunic.  This  is  composed  of  thick, 
firm,  white  fibrous  tissue,  beginning  at  the  optic  nerve  and  ex- 
tending some  four-fifths  of  the  eyeball,  to  the  circumference  of 
the  cornea.  It  gives  form  and  shape  to  the  eyeball,  being 
known  as  the  sclera. 

Considering  the  eye  as  a  camera,  the  inner  coat,  or  retina, 
may  be  regarded  as  the  sensitive  film,  the  middle  ct>at  as  the 
cut-oflf  and  shadow  ajjparatus,  and  the  outer  c»)at,  c.r  sclera,  as 
the  dark  box.  Detailed  descriptions  of  each  of  these  elements 
of  the  eye  will  be  given  later  on. 

CAPSULE  OF  TENON. 
The  larger  part  of  the  eyeball  is  covered  by  a  delicate  mem- 
brane, from  the  optic  nerve  to  within  a  few  tnilllnieters  of  1  »e 


ANATOMY  OF  THE  EYE  33 

corneal  ring-,  called  the  Capsul'e  of  Tenon ;  it  is  also  known  as 
Bonnet's  capsule. 

CONJUNCTIVA. 

The  anterior  portion  of  the  eyeball,  i.  e.,  the  portion  that  is 
visible,  is  covered  externally  by  a  thin,  transparent  mucus 
membrane,  called  the  Conjunctiva.  This  membrane  is  con- 
tinuously reflected  over  the  eyelids,  so  as  to  form  a  sac.  The 
part  that  covers  the  eyeball  is  known  as  the  bulbar  conjunctiva, 
that  which  lines  the  lids  as  the  palpebral  conjunctiva.  The 
fold  between  the  two  is  called  the  conjunctival  fornix,  upper 
and  lower  respectively. 

An  extremely  thin  layer  of  the  conjunctiva  is  continued  over 
the  cornea,  as  a  protection  to  that  tissue,  and  is  known  as  the 
corneal  conjunctiva. 

SCLERA. 

The  sclera,  or  sclerotic, — the  outer  coat,  which  gives  shape 
and  support  to  the  globe,  is  composed  of  connective  tissue. 
It  is  firm  and  white,  and  relatively  thick,  especially  at  the  back ; 
and  to  it  are  attached  the  extrinsic  muscles  of  the  eyeball. 

CORNEA. 

Set  into  the  front  part  of  the  sclera,  much  as  a  crystal  into 
a  watch,  is  a  transparent  membrane,  called  the  cornea,  form- 
ing- the  front  window  of  the  anterior  chamber.  It  consists 
of  five  layers,  from  without  in  as  follows : 

Epithelium. 

Bowman's  Membrane. 

Cornea  Proper. 

Descemet's  Membrane. 

Endothelium. 

The  cornea  is  not  furnished  with  blood-vessels,  as  these 
would  impair  its  transparency,  but  is  richly  supplied  with 
nerves  from  the  fifth  cranial  and  sympathetics.  Its  function 
is  to  serve  as  a  refracting  medium  for  the  light  which  enters 
the  eye. 

A  detailed  description  of  this  important  membrane  will  be 
found  under  the  heading  of  Cornea. 

The  circumferential  ring  representing  the  junction  of  cornea 
and  sclera  is  called  the  limibus,  or  sclero-corneal  junction.    Im- 


34  ANATOMY  OF  THE  EYE 

mediately   outside   this   ring,   in   the   deep   scleral    tissue,   is  a 
circular  lyniijhalic  canal,  called  the  canal  of  Schlemm. 

CRYSTALLINE  LENS. 

A  few  millimeters  back  of  the  cornea,  within  the  eye.  front- 
ing the  Cornea  in  a  vertical  position,  is  a  small,  transparent 
lentil-sha})ed  body,  the  crystalline  lens.  Its  functicjn,  also,  is  a 
refracti\e  one.  It  is  bi-convex,  the  back  surface  being  more  con- 
vex than  the  front,  and  is  composed  of  concentric  layers  of 
elastic  fibre,  contained  in  a  capsule.  In  youth  its  elasticity  is 
considerable,  enabling  it  to  increase  its  convexity  in  the  act  of 
accommodation,  but  its  elasticity  gradually  diminishes  during 
life  and  the  power  of  accommodation  proi)ortionately  decreases, 
until  both  are  entirely  lost. 

The  lens  is  attached  to,  and  held  in  place  by,  a  fine  circular 
membrane  (wrongly  termed  a  ligament),  formed  by  a  reflec- 
tion of  the  hyaloid  membrane,  known  as  the  zonula  ciliaris. 
It  is  also  called  the  z(jnula  of  Zinn.  and  the  suspensory  liga- 
ment. In  the  folds  of  this  zonula  is  supposed  to  be  a  circular 
canal,  known  as  the  canal  of  Petit.  Like  the  cornea,  the  lens 
has  no  blood  \-essels. 

For  further  details  of  this  structure  see  Lens,  Crystalline. 

ANTERIOR  CHAMBER.     AQUEOUS   HUMOR. 

The  space  between  the  cornea  and  the  crystalline  lens  is 
termed  the  anterior  chamber  of  the  eye,  and  is  filled  with  a 
transparent  fluid,  called  the  acpieous  humor.  The  ac|ueous 
humor  is  virtually  water  with  a  few  salts  in  solution,  whose 
(jptical  density  is  1.333,  and  constitutes  one  of  the  refracting 
media  of  the  eye. 

IRIS. 

Suspended  in  the  a(pu'ous  humor,  just  in  front  of  the  crystal- 
line lens,  is  a  circular  niusculo-\ascular  curtain,  called  the  iris, 
disiding  the  anterior  chamber  into  two  sections.  .*^ome  an- 
atomists call  these  two  divisions  the  anterior  .and  jxisterior 
chamber,  respectively. 

The  iris  is  really  a  portion  of  the  ciliary  body,  from  which 
it  springs.  In  its  centre  is  a  circular  aperture,  called  the  pu[)il, 
through  which  light  iiitirs  the  eye.  It  has  two  sets  of  muscles, 
circidar  .and  radiatiuL;  ;  contraction  of  tlu'  foiinrr  <liniinish  the 


ANATOMY  OF  THE  EYE  35 

size  of  the  pupillary  opening,  contraction  of  the  latter  enlarges 
it.  The  iris  is  richly  supplied  Avith  blood  by  the  ciliary  vessels, 
which  anastomose  v-nth  the  conjvinctival  vessels.  It  is  also  the 
seat  of  pigment  which  gives  the  eye  its  distinctive  color — 
hence  the  name,  iris.  In  health  it  has  a  beautiful  lustre,  which 
is  lost  in  disease. 

The  function  of  the  iris  is  that  of  a  diaphragm,  or  cut-ofif, 
to  regulate  the  amount  of  light  entering  the  eye. 

Around  its  circumferential  edge  it  meets  the  cornea,  the  angle 
thus  formed  being  known  as  the  iritic  angle. 

CILIARY  BODY. 

The  middle  part  of  the  second  tunic  of  the  eye  is  called  the 
ciliary  bod}',  because  of  the  hair-like  appearance  of  .its  striata. 
It  consists  of  the  orbicularis  ciliaris,  which  is  hardly  distin- 
guishable from  the  chorioid,  the  corona  radiate,  a  rufif  around 
the  edge  of  the  iris,  the  ciliary  glands,  and  the  ciliary  muscle. 

The  latter  is  a  sphincter  muscle  surrounding  the  crystalline 
lens  and  attached  to  the  chorioid.  Its  contraction,  according  to 
Helmholz.  draws  forward  the  chorioid,  relaxes  the  tension  of 
the  zonule,  and  allows  the  lens  to  assume  a  more  convex  form 
(accommodation) . 

The  ciliary  is  plentifully  supplied  with  blood  through  the 
ciliary  vessels,  and  with  nerves  from  the  fifth  cranial  and  the 
sympathetics.     (See  Ciliary  Body.) 

VITREOUS. 

The  large  space  between  the  crystalline  lens  and  the  retina, 
constituting  about  four-fifths  of  the  globe,  is  filled  with  a  trans- 
parent, gelatinous  mass,  consisting  of  a  clear  liquid  enclosed  in 
the  meshes  of  a  reticulum,  called  the  vitreous  humor,  because 
of  its  optical  resemblance  to  glass.  In  the  fetus  it  is  traversed 
by  an  artery,  the  hyaloid  artery,  piercing  it  centrally  from 
retina  to  lens,  but  after  birth  this  artery  disappears,  leaving  a 
canal  known  as  the  central  or  hyaloid  canal.  (Occasionally 
the  artery  persists  after  birth.)  In  extra-uterine  life  the  vitre- 
ous is  destitute  of  vessels  and  nerves.  Probably  the  hyaloid 
canal  serves  as  a  lymph  channel. 

The  vitreous  serves  as  a  further  refracting  medium  of  the 
eye,  having  an  optical  densit}-  of  1.336. 


36  ANATOMY  OF  THE  EYE 

RETINA. 

Spread  out  over  the  inner  surface  of  the  choroid  is  a  thin, 
transparent  membrane,  consisting  of  a  flared  continuation  of 
the  optic  nerve,  which  serves  as  the  sensitive  film  for  the  cam- 
era of  the  eye.  This  is  the  retina.  It  is  made  up  of  ten  layers, 
from  within  outward  as  follows: 

Internal  limiting  membrane. 

Fibrous  layer  (nerve  fibres). 

Vesicular  layer  (nerve  cells). 

Inner  molecular  layer. 

Inner  nuclear  layer. 

Outer  molecular  layer. 

Outer  nuclear  layer. 

External  limiting  membrane. 

Layer  of  rods  and  cones  (Jacob's  membrane). 

Pigmentary  layer. 

The  layer  of  rods  and  cones  represents  the  spread-out  ending 
of  the  optic  ner\  e.  A  little  to  the  temporal  side  is  the  small, 
circular  spot,  devoid  of  vessels  but  rich  in  nerve-endings, 
where  vision  is  the  keenest,  known  as  the  yellow .  spot,  be- 
cause of  its  color,  or  the  macula  lutea.  In  the  centre  of  tlie  yel- 
low spot  is  a  still  more  sensitive  spot  called  the  fovea  centralis. 

The  retina  is  attached  to  the  choroid  only  at  the  optic  disc 
and  at  the  anterior  l^order.  At  the  latter  attachment  its  edge 
is  saw-shaped,  and  is  known  as  the  ora  serrata.  It  is  plenti- 
fully supplied  with  blood  through  the  branches  of  the  central 
retinal  artery,  which  enters  with  the  optic  nerve. 

For  further  description,  see  Retina. 

CHOROID. 

Immediately  underneath  the  retina  lies  the  choroid,  or  chori- 
oid,  a  pigmented  and  exceedingly  vascular  membrane  consist- 
ing of  five  layers,  from  within  outward  as  follows: 

Suprachoroid,  or  pigmented  epithelium. 

Layer  of  large  vessels. 

Layer  of  medium-sized  \essels. 

Layer  of  capillaries. 

Lamina  vitrea,  or  lamina  basalis. 

All  but  the  last  two  la\ers  are  richly  supplietl  with  pigment 
substance,  wliicli  j;i\es  the  choroid  its  dark  bixiwii  color,  and 


ANATOMY  OF  THE  EYE  Z7 

serves  to  absorb  the  excess  of  light  falling  on  the  retina.  The 
abundant  blood  supply,  derived  from  the  ophthalmic  and  ciliary 
arteries,  helps  to  nourish  the  retina. 

The  choroid  is  attached  to  the  optic  nerve,  where  the  latter 
enters  the  eye,  and  anteriorly  is  continuous  with  the  ciliary 
body.  Since  the  retina  is  transparent,  it  is  the  chorioid  that 
furnishes  the  color  and  substance  of  the  fundus  picture  seen 
with  the  ophthalmoscope. 

UVEA. 

On  account  of  the  similarity  of  the  dark  sphere,  hanging 
upon  the  optic  nerve  as  upon  a  stalk,  to  a  grape,  the  middle 
tunic  of  the  eye  has  received  the  name  of  uvea,  or  the  uveal 
tract. 

OPTIC  NERVE. 

A  little  to  the  nasal  side  of  the  centre  of  the  retina  is  a  light- 
bufif-colored,  circular  area,  about  1.5  mm.  in  diameter,  where 
the  optic  nerve  enters  the  eye.  It  is  known  as  the  optic  disc, 
or  papilla,  and  is  itself  insensible  to  light  stimulation ;  hence 
it  is  also  called  the  blind  spot.  The  disc  is  one  of  the  chief 
landmarks  of  the  fundus  in  ophthalmoscopy.  (See'  Disc, 
Optic.) 

The  optic  nerve,  consisting  of  fibres  gathered  up  from  the 
two  lateral  halves  of  the  retina,  goes  backward  to  the  optic 
chiasm,  where  the  fibres  from  each  temporal  half  cross  over  to 
the  other  side.  From  that  point  on,  the  lateral  half  of  each 
retina  is  represented  on  the  corresponding  side  of  the  brain. 
Thence  the  tract  proceeds  backward  to  the  quadrigeminal  and 
geniculate  bodies,  where  the  fibres  terminate  and  deliver  their 
impulses  to  a  new  set.  The  new  tract  conveys  the  impulses 
to  the  temporal  lobe  on  each  side,  whence  they  are  relayed  by 
another  set  of  nerve  fibres  to  the  left  frontal  lobe  of  the  brain. 
(See  Optic  Tract.) 

MUSCLES. 

The  eye  has  two  sets  of  muscles:  (1)  the  extrinsic  muscles, 
which  are  on  the  outside  of  the  eyeball,  and  execute  the  rotary 
movements  of  the  globe,  and  (2)  the  intrinsic  muscles,  which 
are  inside  the  eyeball,  and  are  concerned  in  the  visual  function- 
ing of  the  eye. 


38 


ANATOMY  OF  THE  EYE 


Prom   llir.schfeld's   Charts, 
a,   a.    l^evator  palpebrae  superior. — b.     Superior  rectus.- -c.     Inferior  rectus. 
— d.    External  rectus. — e.    Internal  rectus. — f.    Inferior  oblique.    All  these  muscles 
in  the  ocular  globe  are  viewed  in  profile 

All  of  the  extrinsic  muscles  have  their  attachment  on  the 
bony  orbit,  and  are  inserted  in  the  sclera  at  con\enient  points 
for  accomplishing  the  movements  rc(|iiircd  of  them,  as  follows: 

Internal  rectus.  Insertion:  About  5.3  mm.  from  the  inner 
margin  of  the  cornea,  on  a  horizontal  line  drawn  through  the 
centre  of  the  cornea.     Function:     Rotates  the  eyeball  inward. 

External  rectus.  Insertion:  About  7  mm.  from  the  outer 
margin  of  the  cornea  on  a  similar  line.  Function:  Rotates  tiie 
eyeball  outward. 

Superior  Rectus.  Insertion  :  About  7.T'  mm.  from  the  upper 
margin  of  the  cornea,  on  a  vertical  line  drawn  through  the  cen- 
tre of  the  cornea.     Function:    Rotates  the  eyeball  upwaril. 

Inferior  rectus.  Insertion:  About  6.5  mm.  from  the  lower 
margin  of  the  cornea  on  a  similar  vertical  line.  Function  :  Ro- 
tates the  eyeball  downward. 

Superior  obli(|uc.  Insertion:  In  the  vertical  meridian  of  the 
eyeball,  bchiiul  the  c(|uator.  This  muscle  arises  from  the  upper 
and  iiim-r  margin  of  the  orbit,  and  jmsscs  through  a  pulley 
(trochlea),  after  which  the  tciulon  bends  sharply  backward  at 
an  acute  angle,  and  passes  under  the  superior  oblicpie  io  the 
eyeball.  Function:  Rotates  the  eyeball  so  that  the  upper  end 
of  the  vertical  meridian  tilts  inward,  and  abducts  it. 


ANATOMY  OP  THE  EYE 


39 


Inferior  oblique.  Insertion :  On  the  outer  side  of  the  eye- 
ball, behind  the  equator.  This  muscle  arises  from  the  lower 
and  inner  margin  of  the  orbit.  Function :  Rotates  the  eyeball 
in  a  precisely  opposite  fashion  to  the  superior  oblique. 

The  internal  and  external  recti  of  the  two  eyes  act  in  pairs, 
to  rotate  them  inward  and  outward,  respectively,  in  equal  de- 
grees. The  action  of  the  internal  recti  is  modified  by  the  in- 
ferior obliques,  so  that  when  the  eyes  turn  inward  they  also 
turn  downward  obliquely ;  that  of  the  external  recti  is  modified 
by  the  superior  obliques,  so  that  when  they  turn  outward  they 
also  turn  obliquely  upward.  In  conjugate  deviation  of  the 
eyes,  i.  e.  when  both  turn  in  the  same  direction,  one  internal 
and  one  external  rectus  act  in  concert. 

All  the  extrinsic  muscles  are  innervated  by  the  third  cranial 
nerve,  except  the  external  rectus,  which  is  supplied  by  the 
sixth,  and  the  superior  oblique,  which  is  supplied  by  the  fourth 
nerve. 

With  the  exception  of  the  internal  and  external  recti,  whose 
action  is  directly  horizontal,  each  eye-muscle  has  a  main  and  a 
subsidiary  action,  concerning  which  it  may  be  said  that  the 
main  action  increases  as  the  subsidiar}-  decreases,  and  vice 
versa,  and  that  the  main  action  increases  in  one  special  direc- 
tion of  the  gaze. 

Main  Action 


Superior  Rectus 


Inferior  Oblique 


Inferior  Rectus 


Moves  eye  up;  action 
increases  as  eye  is 
turned  out;  becomes 
nil   when   eye   is   turned 


Cloves  eye  up;  action 
increases  as  eye  is 
turned  in;  becomes  nil 
when  eye  is  turned  out. 

Moves  eye  down;  ac- 
tion increased  as  eye  is 
turned  out;  becomes 
nil  when  eye  is  turned 
in. 


Superior  Oblique  Moves  eye  down;  ac- 
tion increases  as  eye  is 
turned  in;  becomes  nil 
when  eye  is  turned  out. 


Subsidiary    Action 

Adducts  eye  and  ro- 
tates vertical  meridian 
inward;  action  in- 
creases  as  eye  is 
turned  in.  Raises  up- 
per lid. 

Abducts  eye  and  ro- 
tates vertical  meridian 
outward;  action  in- 
creases as  eye  is  turned 
out. 

Adducts  eye  and  ro- 
tates vertical  meridian 
out;  action  increases  as 
eye  is  turned  in.  Pulls 
down    lower    lid. 

Abducts  eye  and  ro- 
tates vertical  meridian 
in;  action  increases  as 
ej'e  is  turned  out. 


40  ANATOMY  OF  THE  EYE 

The  internal  and  external  recti  are  perfect  antagonists.  The 
superior  and  inferior  recti  are  antagonists  vertically,  but  syn- 
ergists laterally,  both  adducting  the  eye.  The  superior  and  in- 
ferior obliques  are  antagonists  sagitally,  but  synergists  later- 
ally, both  abducting  the  eye. 

In  addition  to  these  extrinsic  muscles  of  the  eye  itself,  there 
are  the  levator  palpebrae,  which  arises  from  the  orbit  and  is 
inserted  in  the  tarsus  of  the  lid,  and  whose  function  is  to  raise 
the  lid :  and  the  orbicularis  palpebrae,  arising  from  the  superior 
maxillary  bone  an«i  inserted  in  the  tarsus,  whose  function  is  to 
close  the  eyelid.  The  former  is  inner\ated  l)y  the  third,  the 
latter  by  the  seventh,  cranial  nerve. 

The  intrinsic  muscles  are  the  ciliary  muscle  and  the  muscles 
of  the  iris.  The  ciliary  muscle  surrounds  the  crystalline  lens, 
and  is  a  sphinctre  muscle.  Its  contraction  increases  the  con- 
vexity of  the  lens,  thus  performing  the  mechanical  part  of  ac- 
commodation. It  is  inner\ated  by  the  third  cranial  nerve, 
through  the  short  ciliary  fibres  of  the  sympathetic. 

The  iris  has  two  sets  of  muscles,  the  concentric,  or  sphincter, 
and  the  radiating  muscles.  Contraction  of  the  former  draws 
the  iris  toward  the  centre,  like  the  shirr-string,  making  the 
pupillary  opening  smaller;  contraction  of  the  latter  pulls  the 
iris  back  toward  its  circumference,  bunching  it  on  itself,  as 
when  we  draw  a  curtain  back,  making  the  pupillary  opening 
wider.  The  circular  muscles  are  innervated  by  the  third  cra- 
nial nerve;  the  radiating  muscles  by  the  sympathetics. 

NERVES. 

Six  out  of  the  twelve  cranial  nerves  go  to,  and  innervate,  the 
eye  and  its  ap])endages.  All  these  nerves,  with  the  exception 
of  the  optic  nerve,  have  their  origin,  or  nucleus,  in  the  fourth 
ventricle  of  the  brain,  just  above  the  medulla  oblongata  Their 
distribution  and  function  is  as  follows: 

Second  Nerve  (Optic  Xcrve).  Sui)plios  the  retina.  Func- 
tion: Vision.  The  nucleus  uf  this  ner\  c  is  in  the  occii)ita! 
lobe. 

Thinl  \erve  (Motor  Oculi).  Sui)plies  all  the  extrinsic  mus- 
cles of  the  eyeball,  except  the  external  rectus  and  the  itiferior 
oblique.  I'unction  :  Rotation  of  the  globe.  Sui)plies  the  cili- 
ary  muscle.      i'uiution  :      .Accoiiimodation.      Sujiplies   the    iris 


ANATOMY  OF  THE  EYE  41 

muscles.  Function:  Contraction  and  expansion  of  the  pupil. 
Supplies  the  levator  palpebrae.    Function :   Raising  the  eyelid. 

Fourth  Nerve  (Patheticus).  Supplies  the  superior  oblique 
muscle  of  the  eyeball.  Function :  Rotation  of  the  eyeball 
obliquely  upward  and  outward. 

Fifth  Nerve  (Sympathetic).  Supplies  the  eyeball  and  lids. 
Function :  Sensation  of  these  parts.  Also  nutriment  of  the 
eye. 

Sixth  Nerve.  Supplies  the  external  rectus  muscle.  Func- 
tion :     Rotation  of  the  eyeball  outward. 

Seventh  Nerve  (Nerve  of  Facial  Expression).  Supplies  the 
orbicularis  palpebrae  muscle.  Function :  Closing  of  the  eye- 
lid. 

The  third  nerve  does  not  itself  reach  and  supply  the  intrinsic 
muscles,  but  delivers  its  nerve-impulse  through  the  ciliary  and 
otic  ganglia  to  the  short  ciliary  fibres  of  the  sympathetic, 
which  supply  these  muscles. 

BLOOD  VESSELS. 

The  eye  has  three  systems  of  blood  vessels,  viz.,  (1)  The 
retinal  vessels,  (2)  the  uveal,  or  ciliary,  vessels,  and  (3)  the 
conjunctival  vessels. 

(1)  The  retinal  blood  supply  is  derived  from  the  central 
retinal  artery,  which  enters  the  eye  through  the  optic  foramen, 
along  with  the  optic  nerve,  and  divides  into  the  retinal  arteries, 
ending  at  the  ora  serrata.  The  retinal  veins  coalesce  into  the 
central  retinal  vein,  which  also  passes  out  through  the  optic 
foramen. 

This  system  of  vessels  forms  a  terminal  loop,  which  does  not 
anastomose  with  any  other  vessels  of  the  eye ;  hence  if  any- 
thing happens  to  the  central  retinal  arteries  there  is  no  compen- 
sation of  circulation.  Occasionally  there  occurs,  as  an  anato- 
mical anomally,  an  anastomosing  artery  communicating  be- 
tween the  retinal  and  ciliary  systems,  known  as  a  cilio-retinal 
artery. 

(2)  The  uveal,  or  ciliary,  system  is  by  far  the  largest  and 
most  numerous  of  the  three  systems.  Its  posterior  division  is 
derived  from  the  ophthalmic  artery,  which  enters  the  eye  near 
the  posterior  pole  and  divides  into  several  branches.  Most  of 
these  go  at  once  to  the  choroid,  and  are  termed  the  short  pos- 


42  ANATOMY  OF  THE  EYE 

terior  ciliarv  arteries.  Two  of  them,  the  long  posterior  ciliary 
arteries,  run,  one  on  tlie  outer  and  one  on  the  inner  side,  be- 
tween the  choroid  and  the  sclera,  as  far  forward  as  the  ciliary 
muscle,  where  each  divides  into  two  branches;  these  run  con- 
centrically with  the  ciliary  ring  and  unite  with  the  arteries  of 
the  opposite  side  to  form  the  major  circle  of  the  iris,  from 
which  arteries  go  to  the  iris.  Shortly  before  they  reach  the 
iris,  these  anastomose  to  form  the  minor  circle  of  the  iris. 

The  anterior  ciliary  arteries  are  derived  from  the  arteries  of 
the  four  rectus  muscles,  and  perforate  the  sclera  near  the 
corneal  margin  to  help  form  the  major  circle  of  the  iris. 

The  short  posterior  ciliary  arteries,  therefore,  chiefly  supply 
the  choroid,  the  long  posterior  and  the  anterior  the  ciliary  body 
and  the  iris. 

The  veins  of  the  iris  and  of  the  ciliary  body  and  those  of  the 
choroid  are  collected  into  the  vasa  vorticosa ;  some  veins,  how- 
ever, from  the  ciliary  muscles  leave  the  eye  as  anterior  ciliary 
veins,  with  which  the  canal  of  Schlemm  anastomoses. 

(3)  The  conjunctival  system  consists  of  posterior  conjunc- 
tival vessels,  communicating  with  the  anterior  ciliary  vessels, 
which  run  to  meet  them,  forming  the  marginal  loops  of  the 
cornea. 

EYELIDS. 
Two  folds  of  external  skin  project  themselves  over  the  eye- 
ball, to  cover  and  protect  it,  above  and  below.  These  are  the 
eyelids.  The  boundaries  of  the  uj)]ier  lids  are  formed  by  the 
eyebrows,  two  lines  of  bushy  hair;  but  the  lower  lids  merge 
imperceptibly  into  the  skin  of  the  cheek.  The  space  between 
the  two  lids,  when  open,  is  called  the  palpebral  fissure ;  the 
angle  where  they  meet,  the  canthus,  internal  and  external,  re- 
spectively. 

The  skin  covering  the  eyelids  is  very  thin.  Beneath  the  skin 
are  the  muscles  by  which  the  lids  are  moved  (orbicularis  pal- 
pebrarum and  levator  palpebrae)  and  the  cartilaginous  sub- 
stance which  gives  them  shape  (the  tarsus).  On  the  inner  sur- 
face the  lids  are  lined  with  reflective  conjunctiva,  under  which 
are  the  Meibomian  glands  and  sebaceous  glands,  both  of  which 
secrete  lubricant  for  the  lids.  .Mong  the  margins  of  the  lids 
are  hair  follicles  from  which  the  lasiies  grow. 


ANGLE  43 

The  lids,  brows  and  lashes,  together,  constitute  what  are 
known  as  the  tutamina,  i.  e.  agents  of  safety. 

LACHRYMAL  APPARATUS. 

The  lachrymal  gland,  which  secretes  the  tears,  consists  of 
two  parts,  the  superior,  lying  in  the  upper  outer  angle  of  the 
orbit,  and  the  inferior,  lying  under  the  mucous  membrane  of 
the  fornix.  Both  empty  into  the  conjunctival  sac  by  means  of 
ducts. 

The  tears  drain  into  the  lachrymal  sac,  which  opens  from 
the  lower  eyelid,  near  the  inner  canthus,  on  a  slight  prominence 
called  the  puncta  lachrymalis.  Thence  into  the  lachrymal 
canal,  which  runs  through  soft  tissue  to  the  superior  maxillary 
bone,  where  it  becomes  continuous  with  the  bony  lachrymal 
duct,  which  opens  into  the  inferior  meatus  of  the  nose. 
Angle.  There  are  several  special  angles  met  with  in  optics,  prin- 
cipally made  either  by  tangents  to  reflective  and  refractive  sur- 
faces, or  by  various  axes  of  reflecting  and  refracting  systems. 
The  most  common  of  them  are  defined  below. 

Angle  Alpha.  The  angle  made  by  the  intersection  of  the 
visual  axis  and  the  principal  axis  at  the  nodal  point  of  the  eye. 
The  longer  the  antero-posterior  diameter  of  the  eyeball,  the 
smaller  this  angle  appears  to  be,  because  the  further  the  retina 
is  from  the  point  of  intersection,  the  less  is  the  arc  subtending 
the  angle.  Hence,  in  myopia  the  angle  alpha  appears  smaller 
than  normal,  giving  an  appearance  of  internal  deviation,  while 
in  hyperopia  exactly  the  opposite  state  of  afifairs  exists.  This 
constitutes  what  is  known  as  apparent  strabismus. 

Critical  Angle.  The  acute  angle  of  incidence  (see  below) 
beyond  which  light  does  not  pass  from  a  dense  into  a  rarer 
refracting  medium,  but  is  totally  reflected  from  the  surface. 
Beyond  this  point,  of  course,  reflection  is  the  most  brilliant 
possible.  The  critical  angle  between  water  and  air  is  48°  35' ; 
between  glass  and  air  41°  48'. 

Angle  Gamma.  The  angle  formed  by  the  intersection  of  the 
principal  axis  of  the  eyeball  with  the  visual  angle  at  the  centre 
of  rotation  of  the  eyeball.  This  angle  has  no  particular  work- 
ing value  in  optometry. 

Angle  of  False  Projection.  When  light  from  an  object  is 
refracted  by  a  prism,  there  is  an  apparent  displacement  of  the 


44  ANGLE 

image  toward  the  apex  of  the  prism.  (See  Prism.)  The  angle 
formed  by  the  visual  axis,  projected  to  the  false  image,  with 
the  incident  ray  is  called  the  angle  of  false  projection. 

Angle  Kappa.  The  angle  between  the  line  of  fixation  and 
the  normal  to  the  cornea  which  passes  through  the  centre  of 
the  cornea. 

Angle  of  Convergence.  The  angle  made  by  the  two  visual 
axes  with  each  other  when  the  eyes  are  turned  inward.  It 
varies,  of  course,  with  the  degree  of  convergence,  being  most 
obtuse  at  the  point  of  greatest  convergence. 

Meter  Angle.  The  angle  made  by  each  visual  axis  with  the 
median  perpendicular  when  the  eyes  are  turned  inward.  It 
varies,  of  course,  with  the  degree  of  convergence,  and  is  exactly 
half  the  angle  of  convergence  (see  above).  It  is  used  more 
than  the  angle  of  convergence  in  calculating  problems  of  con- 
vergence. 

Prism  Angle.  The  angle  made  by  the  two  refracting  sides 
of  a  prism  at  its  apex. 

Angle  of  Deviation.  The  angle  made  by  a  ray  of  light,  after 
passing  through  a  prism  and  being  refracted  by  it,  with  tiie 
path  of  the  incident  ray,  both  being  projected  to  meet  each 
other. 

Angle  of  Incidence.  The  angle  made  by  a  ray  of  light,  as  it 
strikes  a  reflecting  or  refracting  surface,  with  the  perpendicu- 
lar to  that  surface. 

Angle  of  Reflection.     The  angle  made  by  a  reflected  ray  of 
light  with  the  perpendicular  of  the  reflecting  surface.     In  re- 
.    flection  the  angle  of  incidence  and  the  angle  of  rcllection  are 
always  equal. 

Angle  of  Refraction.  'Jhc  angle  made  by  a  ray  of  light,  as 
it  emerges  from  a  refracting  medium,  with  the  i)erpendicular 
to  the  surface  from  which  it  emerges.  This  angle,  of  course, 
is  by  no  means  always  etjual  to  the  angle  of  incidence,  but 
varies  with  the  index  of  refraction.  In  fact,  the  ratio  between 
the  sines  of  the  angles  of  incidciicc  and  refraction  measures 
the  index  of  refraction. 

Visual  .'\ngle.    Tlie  aii^le   I'ormtd  at  tlie  nodal  point  ot   the 
eye  between  two  lini-s  drawn  from  the  yellow  spot  to  the  ex- 
treme boundaries  of  the  object  viewed.     It  is  the  size  of  this 


ANGULUS  45 

angle  which  is  the  principal  factor  in  judging  of  the  size  of 
the  object,  and  also  its  distance  away.  The  further  away  an 
object  is,  the  smaller  visual  angle  it  subtends.  (This  consti- 
tutes what  is  known  as  perspective.)  The  smallest  visual 
angle  which  an  object  can  subtend  and  be  discerned  is  1 
minute ;  this  is  called  the  minimum  visual  angle. 
(See  ACUITY.) 

Angulus.     An  angle.    See  Angle. 

Anianthinopsy.     A'iolet-blindness. 

Aniridia.     Absence  of  the  iris. 

Aniscoria.    Inequality  of  the  size  of  the  two  pupils. 

Anisometropia.  A  difiference  in  the  refraction  of  the  two  eyes, 
so  that  one  focuses  neutral  waves  behind  the  retina  (hyper- 
opia) and  the  other  in  front  of  the  retina  (myopia).  Usually 
it  is  congenital,  in  which  case  the  person,  as  a  rule,  uses  the 
hyperopic  eye  for  distant  vision  and  the  myopic  eye  for  near 
vision. 

Annexa.  A  general  term  applied  to  parts  which  are  adjacent 
and  subservient  to  an  organ.  The  annexa  of  the  eye  are  the 
lids,  lashes,  eyebrows,  etc. 

Annulus  Cilliaris.  The  ring-like  boundary  between  the  iris  and 
the  chorioid. 

Anopsia.  Disvise  of  the  eye.  The  most  common  instance  of 
anopsia  is  where  the  patient  has  diplopia,  and  in  order  to  rid 
himself  of  the  annoyance  of  "seeing  double"'  learns  to  disre- 
gard the  image  in  one  of  his  eyes.  This  eventually  results  in 
an  inability  to  see  with  the  disused  eye,  known  as  amblyopia 
ex  anopsia.    There  are,  however,  other  causes  for  anopsia. 

Anophthalmia.     Absence  of  the  eyes. 

Anotropia.     Turning  of  the  eye  upward.     See  Strabismus. 

Antagonism  of  the  Visual  Field.  When  the  images  of  the  two 
visual  fields  are  of  such  a  nature  that  they  cannot  be  fused,  one 
or  other  of  the  visual  fields  will  predominate  to  the  exclusion 
of  the  other.     It  is  impossible  to  say  which  will  predominate. 


46  ANTERIOR 

Usually  one  predominates  until  some  slight  motic»n  of  the  eye 
changes  the  situation,  when  the  other  will  predominate.  (See 
Binocular  Vision.) 

Anterior.     Toward  the  front. 

Anterior  Chamber.  The  chamber  of  the  eye  lying  in  frtjut 
of  the  crystalline  lens,  containing  the  a(|ueous  humor. 

Anterior  Focal  Point.  The  focal  point  of  a  lens  lying  in 
front  of  the  entering  surface. 

Anterior  Pole.  The  point  where  the  anterior  surface  of  a 
lens  cuts  the  principal  axis. 

Anterior  Staphyloma.  Bulging  of  the  anterior  surface  of  the 
eye,  i.  e.  of  the  cornea. 

Anterior  Synechia.     Adhesion  of  the  iris  to  the  cornea. 

Anticloudine.  A  preparation  for  preventing  the  condensation  of 
steam  or  vapor  on  glass  lenses. 

Aperture.  Technically  this  term  is  used  in  problems  connected 
with  a  refracting  lens  system  to  denote  the  width  of  the  angle 
made  with  the  principal  axis  by  rays  of  light  from  an  object- 
point.  Ordinarily,  the  nearer  the  object-point  is  to  the  lens  the 
wider  is  the  aperture.  We  can,  however,  vary  the  aperture  in 
any  given  problem  b\'  limiting  the  number  of  divergent  rays 
we  deal  with. 

Apex.  The  point  of  meeting  of  two  sides  of  a  triangle.  In 
optics,  its  chief  ajjplication  is  to  a  ])rism.  The  ai)ex  angle  of 
a  i)rism  is  the  angle  made  l)y  the  two  sides,  where  they  meet. 

Aphakia.  Absence  of  the  crystalline  lens,  either  congenital  or 
f(jll(jwing  extraction. 

Aphose.  A  subjectixe  sensation  of  a  dark  spot  in  the  line  of 
\  isiijn. 

Aplanatic.  .A  term  applied  to  reflectors,  lenses,  and  combination 
of  them,  which  are  cai)abk'  of  transmitting  light  with  neither 
spherical  nor  chromatic  aberration.  Usually  accomplished  by 
combining  substances  of  different  ojjtical  density  and  other 
(jualities,  which  counteract  each  other's  aberration. 


APOCHROMATIC  47 

Apochromatic.  A  term  applied  to  an  improved  achromatic  lens, 
which  does  away  with  the  secondary  spectrum  in  the  ordinary 
achromatic  lens,  and  which  corrects  spherical  aberration  for 
two  colors  instead  of  only  one. 

Apparent  Stiabismus.  An  appearance  of  squint  in  high  hyper- 
opia or  myopia,  due  to  the  smallness  or  largeness  of  the  angle 
alpha  (q.  v.). 

Aprosexia.     Lack  of  attention,  due  to  ocular  defects. 

Aqueous  Humor.  The  transparent,  watery  fluid  which  fills  the 
space  in  the  eye  between  the  cornea  and  the  crystalline  lens. 
The  iris  floats  in  this  fluid,  dividing  the  space  into  the  anterior 
and  posterior  chambers.  It  is  really  nothing  but  water  with  a 
few  salts  in  solution;  of  albumin  it  contains  only  a  faint  trace. 

It  is  generally  agreed  that  the  aqueous  humor  is  secreted 
mostly  by  the  ciliary  body,  a  little  being  secreted  by  the  iris. 
That  which  comes  from  the  iris  leaves  the  chamber  easily  by 
way  of  the  ligamentum  pectinatum  ;  that  which  is  derived  from 
the  ciliary  body  has  no  such  direct  outlet.  Since  a  greater 
quantity  is  secreted  by  the  ciliary  than  by  the  iris,  the  flow  is 
forward,  toward  the  ligamentum  pectinatum,  by  which  it  es- 
capes ;  perhaps  this  flow  is  continuous,  perhaps  periodic. 
Occlusion  produces  intra-ocular  tension,  as  seen  in  glau- 
coma. The  quantity  of  aqueous  humor  secreted  was  formerly 
greatly  exaggerated,  observers  being  misled  by  the  fact  that 
when  the  chamber  is  emptied  by  puncture  the  fluid  regenerates 
itself  and  fills  the  chamber  again  in  about  six  minutes.  This 
fluid,  however,  is  not  the  perfectly  organized  humor;  it  is  rich 
in  albumins,  and  contains  the  protective  bodies  (alexins)  of  the 
blood  which  defend  against  infection. 

As  a  refracting  medium  the  aqueous  humor  has  an  index  of 
1.336,  which,  being  practically  the  same  as  that  of  the  cornea, 
prevents  it  from  being  an  active  factor  in  refracting  the  light 
which  enters  it  from  the  posterior  surface  of  the  cornea. 

Arcus.  An  arc-shaped  cloudy  formation  on  the  cornea  or  the 
crystalline  lens. 

Arcus  Senilis  Corneae.  A  cloudiness  which  appears  in  old 
age  near  the  corneal  margin,  showing  first  as  two  arcs,  on  the 


48  ARCUS   TARSEUS 

upper  and  lower  margins  respectively,  which  presumably  coa- 
lesce to  form  a  ring. 

Arcus  Senilis  Lentis.  A  ring-like  opacity  found  in  old  people 
near  the  ec|uator  of  the  lens.  It  does  not  interfere  with  vision, 
as  it  lies  wdiolly  behind  the  iris,  and  it  shows  no  disposition  to 
spread. 

Arcus  Juvenilis.  Occasionally  the  above  phenomena  occur 
in  young  people,  in  whicii  e\ent  they  are  called  arcus  juvenilis. 

Arcus  Tarseus.  The  two  arterial  arches  from  which  the  con- 
junctiva of  the  upper  eyelid  obtains  its  blood  supply.  They  are 
known  as  the  superior  and  inferior  arches,  respectively. 

Argamblyopia.     Another  word  for  amblyopia  ex  anopsia. 

Argyll  Robertson  Pupil.  A  condition  in  which  the  pupil  does 
not  contract  when  light  is  thrown  upon  the  retina,  but  con- 
tracts under  accommodation.  It  occurs  in  diseases  of  the  cen- 
tral nervous  system,  such  as  locomotor  ataxia,  myelitis,  etc. 
(See  Reflex.) 

Argyrol.  An  organic  salt  of  silver,  extensively  used  as  a  colly- 
rium  in  inflammations  of  the  eye.  It  is  employed  in  solutions 
of  5%  to  25%  in  distilled  water,  and  is  much  safer  and  better 
than  nitrate  of  silver. 

Argyrosis.  Black  or  grey  discoloration  of  the  skin  or  a  mucous 
membrane,  due  to  the  continued  use  of  silver  salts,  particularly 

nitrate  of  siher. 

Arteries.  All  the  arteries  of  the  eye  and  the  optic  tract  are  de- 
rived from  the  internal  carotid  artery.  This  artery,  so  far  as 
these  organs  are  concerned,  gives  off  three  subsidiary  arteries 
two  of  which,  the  chorioidal  and  the  posterior  communicating, 
go  to  the  optic  tract,  and  one,  the  ophthalmic,  goes  to  the  eye- 
ball, dividing  into  the  numerous  branches  which  supply  the 
globe.  The  arterial  s\stem.  therefore,  is  as  follows: 
Chorioidal.  Ojjtic  tract. 

Posterior   Communicating.          ( )j)tic  tract. 
Central  Retinal.  Retina. 

Short  Posterior  Ciliary.  Chorioid. 

Long  Posterior  Ciliary.  Chorioid  and  Ciliary  Hody. 


ARTIFICIAL    EYE 


49 


Conjunctival. 

Frontal. 

Lacrymal. 


Anterior  Ciliary. 

Internal  Maxillary. 
Infraorbital. 


Conjunctiva. 
Supraorbital  region. 
Lacrymal  gland. 
External  rectus  muscle. 
Superior  Rectus  Muscle. 
Upper  eyelid. 
Conjunctiva. 
Ciliary  Body. 


Inferior  rectus  muscle. 
Inferior  oblique  muscle. 
Lacrymal  gland  and  sac. 
Lower  eyelid. 
The  zygomatico-orbital  artery,  having  its  origin  in  the  tem- 
poral artery,  supplies  the  orbicularis  palpebrarum  muscle. 

Artificial  Eye.  A  glass  eye,  made  to  resemble  the  natural  eye, 
to  be  inserted  in  the  orbit  to  take  the  place  of  an  eye  which  has 
been  enucleated.  These  eyes  are  nowadays  made  with  great 
art.  They  usually  last  about  a  year,  after  which  they  lose 
their  smoothness  and  are  apt  to  irritate  the  socket. 


Artificial  Eye. 

As.     An  abbreviation  for  astigmatism. 

Associated  Movements.  In  physiology  this  term  refers  to  mus- 
cular movements  which,  while  they  are  not  dependent  upon 
each  other,  are  instinctively  performed  simultaneously.  A 
familiar  and  ludicrous  example  are  the  movements  of  the 
tongue  with  which  a  child  often  accompanies  the  act  of  writ- 
ing. A  more  sensible  one  is  the  swinging  of  the  arms  when 
walking.  No  doubt  the  physiologic  relation  between  accom- 
modation and  convergence  is  that  of  associated  movements. 

Asthenopia.  This  is  a  broad  term  used  to  designate  a  group  of 
symptoms  arising  from  any  form  of  functional  eye-strain,  in- 
cluding that  type  of  muscular  and  nervous  exhaustion  due  to 


50  ASTHENOPIA 

licterophoria.  In  a  general  way.  these  symptoms  are  the  same, 
from  \vhate\  er  specific  cause  they  arise,  and  in  the  last  analy- 
sis they  arc  essentially  rptlex  in  their  character,  dependent 
upon  an  excessive  or  une(|ual  innervation,  and  mediated  pri- 
maril}-  through  the  ocular  and  fifth  cranial  nerxes. 

SYMPTOMS. 

Asthenopia  manifests  itself  in  an  inahilit}'  to  sustain  a 
steady  or  ])rolonged  use  of  the  eyes,  and  by  more  or  less  pain 
and  discomfort  when  this  is  attempted.  The  pain  may  be  of 
any  degree  of  severity,  e\  en  to  an  agonizing  neuralgia.  At 
times  there  is  no  actual  ])ain  at  all.  but  after  a  prolonged  use 
of  the  eyes  the  vision  becomes  l)lurrcd  and  the  patient  is 
obliged  to  rest. 

If,  in  si)ite  of  the  inconxenience.  the  j^atient  forces  himself  to 
continue  his  work,  he  will  presently  begin  to  de\elop  positive 
signs  of  inflammation,  such  as  photophobia,  conjunctivitis,  etc. 
Headache  is  a  prominent  symptom  of  asthenopia,  and  assumes 
\arious  forms.  Hyperopia  usually  gives  brow  headache,  simi- 
lar to  that  of  nasal  catarrh.  Astigmatism,  pain  in  the  back  of 
the  neck,  with  tender  spots  ui)on  the  head. 

The  organs  and  functions  most  closely  associated  with  the 
ocular  nerves  are  those  of  digestion ;  hence  the  commonest 
systemic  disturbances  resulting  from  eye-strain  arc  those  of 
stomach  and  bowel  disorders;  lack  of  appetite,  dyspepsia,  nau- 
sea, vomiting,  constipation,  etc.  Next  to  these  come  disturb- 
ances of  the  general  nervous  system,  such  as  chorea,  bed- wet- 
ting, insomnia,  and  even  epilepsy.  Dizziness  is  always  a  more 
or  less  noticeable  accompaniment  of  eye-strain,  because  it  is  by 
ihe  imconscious  estimate  we  make  of  the  amount  of  ner\e  en- 
erg}-  expended  in  accommodation  and  con\ergence  that  we 
largely  make  our  visual  judgments;  and  when  these  facultie.s 
are  unnaturall}-  exercised  di//iness  results. 

VARIETIES. 

A  common  foi  ni  of  asthenopia  is  that  which  results  from 
muscular  iml)a]ance.  by  reason  of  the  une(|ual  or  excessive  in- 
nervation performed  by  the  patient  in  order  to  obtain  single 
A'ision.  Tiiis  type  is  known  as  muscular  astlu'uopi.i.  indicating 
that  its  seat  is  in  the  extrinsic  ocular  muscles. 


ASTHENOPIA  .51 

A  second  form  of  asthenopia,  which  really  also  belongs  in 
the  muscular  class,  is  th.at  which  arises  from  straining  of  the 
ciliary  muscle  in  cases  of  hyperopia  or  hyperopic  astigmatism. 
For  the  sake  of  distinction  from  that  which  depends  upon  the 
extrinsic  muscles,  however,  this  variety  is  usually  termed 
accommodative  asthenopia.  It  is  by  far  the  commonest  form 
of  the  trouble,  as  hyper(.)pia  and  hyperopic  astigmia  are  the 
commonest  errors  of  refraction.  Furthermore,  it  is  the  type 
that  gives  most  trouble  to  the  refractionist,  because  it  con- 
notes a  ciliary  spasm. 

A  third  and  rarer  type  of  asthenopia  is  that  which  comes 
from  hyper-sensitiveness  or  fatigue  of  the  retina,  either  due  to 
close  and  continuous  work  in  a  bright  light,  or  over  glittering 
materials  or  to  constitutional  diseases.  This  is  called  retinal 
asthenopia. 

REFLEX  EFFECTS. 

It  is  well  known  that  the  muscular  activities  of  the  body  are 
under  the  control  of  the  nervous  system.  It  is  further  known 
that  this  great  system  of  nerve  fibres  and  centres  crosses  and 
recrosses,  exchanging  small  connecting  fibrils,  like  some  vast 
telephone  or  telegraph  system,  with  "crossed  wires,"  so  that  all 
the  centres  and  out-posts  (peripheries)  are  intimately  con- 
nected with  each  other.  Not  one  is  isolated  or  independent  of 
another,  and  while  a  nerve  current  may  be  sent  by  a  special 
centre  along  a  special  nerve  path  to  a  group  of  muscles  to- per- 
form a  special  act,  yet  every  nerve  centre  and  every  organ  in 
the  body  shares  to  some  extent  in  the  nervous  discharge. 

In  the  furnishment  of  this  motive  control  by  the  nerve  cen- 
tres under  normal  conditions  there  is  an  economic  regulation  of 
the  amount  of  nerve  energy  furnished,  in  accordance  with  the 
amount  of  work  to  be  done.  Especially  is  this  the  case  where 
very  fine,  delicate  movements  are  to  ])e  made,  requiring  nicety 
of  judgment.  Usually,  for  such  acts,  two  sets  of  muscles  have 
to  be  employed,  one  set  opposing  the  other,  and  the  motive 
force  supplied  to  each  set  is  nicely  balanced  so  as  to  give  pre- 
cisely the  needed  resultant  effect.    This  is  called  co-ordination. 

The  functions  of  accommodation  and  convergence  both  be- 
long to  this  class.  Under  normal  conditions,  precisely  the 
same  amount  of  nerve  discharge  is  required  for  the  extrinsic 


•^2  ASTIGMATISM 

muscles  of  each  eye  to  hold  the  eyes  balanced  in  proper  con- 
vergence, and  the  same  for  each  ciliary  to  maintain  equally 
focussed  images  on  the  retinae.  But  suppose  that  the  refrac- 
tion of  the  two  eyes  is  different;  then,  in  order  to  satisfy  the 
demand  for  clear  vision,  one  ciliary  must  contract  more  than 
the  other;  an  unnatural  and  unequal  innervation  must  be  per- 
formed. In  other  words,  co-ordination  becomes  an  unnatural 
and  extraordinary  effort. 

Again,  suppose  that  the  conditions  of  convergence  are  ab- 
normal. The  exercise  of  accommodation,  as  we  know,  is  a 
normal  and  powerful  stimulus  to  convergence.  Hence,  when, 
as  in  hyperopia  or  myopia,  the  normal  relation  between  the 
two  functions  is  disturbed,  the  natural  tendency  of  convergence 
is  to  follow  suit,  which  it  is  prevented  from  doing  only  by  an 
unnatural   innervation  of  the   muscles   concerned. 

In  either  of  these  abnormal  conditions,  then,  the  nervous 
efifort  required  to  balance  the  muscles  to  the  desired  resultant 
of  function  is  unnatural  and  extraordinary.  But,  as  already 
explained,  the  whole  nervous  organization  of  the  body  shares 
in  the  nervous  discharge  of  any  one  function,  and  this  is  true 
to  a  special  degree  in  the  case  of  vision,  which  is  so  essential 
a  factor  in  every  act,  emotion,  or  thought.  Hence  any  un- 
natural and  extraordinary  exercise  of  the  nervous  mechanism 
of  vision  cannot  but  make  itself  very  quickly  felt  in  an 
unnatural  and  extraordinary  nervous  impression  upon  other 
organs  and  functions.  Thus  it  is  that  many  cases  of  digestive 
and  nervous  disease  are  caused  by  eye-strain,  and  may  be 
cured  by  the  proper  fitting  of  glasses,  whicli  correct  the  refrac- 
ti\e  errors,  render  the  eyes  normal,  and  ecpialize  inner\  ation. 

Astigmatism.  The  word  literally  means  "without  a  point."  It 
denotes  a  condition  of  the  eye  in  which  waves  of  light  are  not 
focussed  at  one  point,  as  in  the  normal  eye,  but  at  two  sep- 
arate focal  points.  This  is  due  to  the  fad  th.it  the  curxature 
of  one  of  the  refracting  surfaces  of  the  eye — usually  the  ctjrnea. 
but  sometimes  the  crystalline  lens — instead  of  being  spherical, 
is  jjaraboloid,  i.  c.  nh-ide  up  ol  two  dilTerent  spherical  cur\es 
intersecting  each  other  at  right  angles;  and  each  of  these 
curvatures  has  its  own  set  of  focal  points,  those  of  the  most 
convex  curxature  Iving  more  anU-rior  than   tluise  of  tlu-  least 


ASTIGMATISM 


53 


convex.  Under  such  conditions,  it  is  evident,  a  clear  retinal 
image  cannot  be  obtained  of  any  object,  since  the  two  sets  of 
foci  cannot  be  upon  the  retina  at  the  same  time.  Hence 
astigmatism  is  incompatible  with  clear  vision. 

The  cause  of  astigmatism  is  a  much  mooted  question.  The 
normal  eye  is  slightly  astigmatic,  being  a  trifle  more  convex 
in  the  vertical  meridian  than  in  the  horizontal ;  not  enough, 


Diagram  of  Astigmatism,  the  Vertical  Meridian  focussing  at  P,  and  the 
horizontal  meridian  at  F^,  the  relative  positions  of  P  and  P^  with  regard  to  the 
letinai  plane  determine  the  kind  of  Astigmatism. 

however,  to  interfere  noticeably  with  clear  vision.  Some 
attribute  this  to  pressure  of  the  eyelids ;  others  to  the  action 
of  the  extrinsic  muscles.  If  these  theories  be  true,  then  cases 
of  abnormal  astigmatism  can  hardly  be  regarded  as  exaggera- 
tions of  this  normal  irregularity  in  curvature,  for  while  they 
might  account  for  corneal  astigmatisms  in  which  the  greatest 
convexity  is  vertical,  they  would  not  explain  the  opposite  type 
of  corneal  astigmatism,  or  any  kind  of  lenticular  astigmatism. 
The  strong  probability  is  that  all  cases  are  congenital,  and 
depend  upon  some  defect  in  biological  development. 

VARIETIES. 

As  stated,  the  great  majority  of  astigmatisms  are  corneal, 
i.  e.,  due  to  paraboloid  curvature  of  the  surface  of  the  cornea ; 
some  five  to  ten  per  cent  are  lenticular,  i.  e.,  due  to  paraboloid 
curvature  of  the  crystalline  lens. 

Irregular  Astigmatism.  Occasionally  cases  of  astigmatism 
are  met  with  in  which  the  two  intersecting  curves  are  not  at 
right  angles  to  each  other,  but  intersect  obliquely ;  and  still 
others  in  which  there  are  several  dififerent  curvatures,  lying 
at  various  angles.  Such  cases  are  known  as  Irregular  Astig- 
matism, and  are  usually  due  either  to  injury  (corneal  ulcers, 
wounds,  etc.)  or  to  operations  on  the  eye  (cataract  extractions, 
iridectomies,  etc.).  The  detection  and  management  ot  these 
cases  will  be  dealt  with  in  a  separate  fashion. 


54.  ASTIGMATISM 

Regular  Astigmatism.  By  far  tlie  vast  majority  of  cases 
of  astigmatism  i>re  regular,  i.  e.,  they  exhibit  hut  two  different 
curvatures,  which  intersect  each  other  at  right  angles.  The 
two  meridians  of  greatest  and  least  curvature,  respectively,  are 
known  as  the  principal  or  chief  meridians.  Only  in  these  two 
meridians  is  the  curvature  spherical ;  in  intermediate  meridians 
there  are  differing  grades  of  paraboloid  curvature. 

When  the  two  chief  meridians  are  vertical  and  horizontal, 
respectively,  the  condition  is  said  to  be  a  right  astigmatism  ; 
when  they  are  at  other  angles,  it  is  termed  an  oblique  astig- 
matism. Further,  when  the  meridians  of  greatest  and  least 
curvature  follow  the  order  exhibited  in  the  normal  eye,  name- 
ly, the  greatest  curvature  vertical  and  the  least  curvature 
horizontal,  we  say  that  the  astigmatism  is  "with  the  rule"; 
when  this  order  is  reversed,  we  say  it  is  "against  the  rule." 

From  an  optical  standpoint,  there  is,  strictly  speaking,  but 
one  kind  of  astigmatism,  the  essential  element  of  the  condi- 
tion being  that  one  principal  posterior  focus  lies  in  a  more 
anterior  plane  than  the  other,  or  (what  is  the  same  thing)  one 
lies  in  a  more  posterior  plane  than  the  other.  Whatever  other 
classification  we  give  to  cases  of  astigmatism  really  i)ertains 
to,  and  depends  upon,  co-existing  conditions  of  emmetropia. 
hyperopia  or  myopia.  For  clinical  convenience,  how'ever,  we 
classify  astigmatism  according  to  the  relative  positions  of  the 
two  posterior  principal  foci  with  reference  to  the  retinal  plane : 

1.  Simple  Ilyperopic  Astigmatism,  where  one  posterior 
principal  f(jcus  lies  in  the  retinal  plane  and  the  c4her  posterior 
to  it. 


i..»fl.H,,...^.  ^.tf 


2.     C(jm])oun(l    liyperoi)ic   .Astigmatism,     wlu-rc    ])oth    pos- 
terior principal  points  lie  posterior  to  llu'  retinal  pl;ine. 


c.^-,*  Sv/  ' 


ASTIGMATISM  55 

3.     Simple  Myopic  Astigmatism,  where  one  posterior  prin- 
cipal focus  lies  at  the  retinal  plane  and  the  other  anterior  to  it. 


5.™;(t.   /)y. 


4.     Compound   Myopic  Astigmatism,  where  both  posterior 
principal  foci  lie  anterior  to  the  retinal  plane. 


5.     Mixed  Astigmatism,  where  one  posterior  principal  focus 
lies  anterior,  and  the  other  posterior,  to  the  retinal  plane. 


Next  to  hyperopia,  (with  which  it  is  frequently  associated), 
astigmatism  is  the  most  frequently  encountered  of  refractive 
errors.  And,  as  hyperopia  is  the  most  frequent  of  the  spherical 
errors,  so  hyperopic  astigmatism  is  the  most  frequently  met 
type  of  astigmatism. 

CYLINDERS  IN  ASTIGMATISM. 

Fortunately,  astigmatism,  once  it  is  detected  and  measured, 
is  very  easily  and  accurately  corrected,  by  means  of  a  lens 
made  from  a  segment  of  a  cylinder.  (See  Lens.)  And  as 
cylindrical  lenses  also  play  the  major  role  in  subjective  tests 
for  astigmatism,  it  will  be  well  to  point  out  in  this  place  their 
application  to  this  refractive  error. 

A  cylindrical  lens  has  been  elsewhere  described  as  being, 
optically,  the  split  half  of  a  sphere.  By  placing  it  before  the 
eye,  therefore,  we  can  add  to  (with  a  convex  cylinder)  or  sub- 
tract from  (with  a  concave  one)  the  curvature  of  the  eye  in  one 
meridian — the  meridian  at  right  angles  to  the  axis  of  the  cylin- 
der— without  changing  the  power  of  the  opposite  meridian.    If, 


56  ASTIGMATISM 

therefore,  a  cylinder  of  the  proper  degree  of  curvature,  plus  or 
minus,  be  placed  before  the  eye  with  its  axis  at  right  angles 
to  the  chief  meridian  which  it  is  designed  to  influence,  it  will 
move  the  principal  focal  point  of  that  meridian  backward  or 
forward,  as  the  case  may  be,  until  it  is  coincident  with  the 
focal  point  of  the  other  meridian.  Both  chief  meridians  now 
focussing  alike,  the  refraction  of  the  eye  is  that  of  a  sphere; 
astigmatism  has  ceased  to  exist.  If  the  amalgamated  focal 
point  is4iot  in  the  retinal  plane,  a  spherical  lens  of  the  proper 
curvature  and  power  will  put  it  there. 

Plainly,  then,  the  correcting  cylinder  in  astigmatism  must 
always  be  equal  in  dioptrism  to  the  difference  between  the  two 
chief  meridians.  \\'hether  it  be  plus  or  minus  is  immaterial, 
for  it  matters  nothing  which  of  the  two  focal  points  is  moved 
so  as  to  coincide  with  the  other.  The  axis  of  the  cylinder  must 
be  across  the  meridian  it  is  intended  to  infJULMice,  as  the  power 
of  a  cylinder  is  at  right  angles  to  its  axis. 

DETERMINATION   OF   ASTIGMATISM. 

Subjective  determination  of  astigmatism  depends  upon  the 
fact  that  it  is  impossible  to  see  clearly,  at  the  same  time,  in 
both  meridians  of  the  visual  field  corresponding  to  the  principal 
meridians  of  curvature.  All  sulijective  tests  are  designed  (a) 
to  narrow  these  contrasting  meridians  down,  and  to  emphasize 
their  contrast,  and  (b)  to  find  the  lens  power  which  will  focus 
them  both  on  the  retina  simultaneously,  and  gi\e  clear  spherical 
^•ision.  Since  astigmatism  is  a  static  condition,  depending  upon 
the  curvature  of  the  cornea  or  lens,  it  is  tested  for  with  the 
eye  in  a  static  condition,  i.  e.,  at  infinity. 

The  simplest  test  is  that  afforded  by  the  astigmatic  wheel 
chart,  consisting  of  a  radiating  series  of  black  lines,  radii  of 
a  circle,  corresponding  to  the  various  angular  degrees.  It  is 
evident  that  two  of  these  lines,  at  right  angles  to  each  other, 
will  represent  the  patient's  two  chief  meridians  of  refraction, 
and  he  will  see  one  of  these  lines  more  clearly  than  all  the  rest 
and  the  opposite  one  least  clearly  of  all.  If  we  now  find  a  cyl- 
inder which,  properly  placed,  will  make  all  the  lines  appear 
equally  black,  we  shall  ha\e  made  the  two  chief  nuridians 
cfjual,  and  corrected  the  astigmatism. 


ASTIGMATISM 


57 


There  are  several  possible  sources  of  error  in  this  rather 
crude  method ;  for  one  thing,  the  difference  between  the  two 
chief  meridianal  lines  is  often  hard  to  distinguish  when  all 
the  intermediate  lines  are  in  full  view ;  and;  besides,  if  the 
patient  happen  to  have  a  mixed  astigmatism,  in  which  the  two 
chief  meridians  are  about  equally  hyperopic  and  myopic,  re- 
spectively, there  will  be  no  noticeable  difference  in  the  two 
meridianal  lines. 


Astigmatic  Wheel. 

To  make  the  test  more  delicate  and  dependable,  it  is  better 
to  "fog,"  i.  e.,  put  a  strong  plus  lens  before  the  tested  eye  which 
blots  the  chart  out  altogether,  and  then,  by  gradually  adding 
minus  pov^er,  gradually  move  the  principal  focal  points  of 
the  two  chief  meridians  back  until  one  of  them  (that  of  least 
curvature)  falls  on  the  retina.  At  once  one  of  the  lines  will 
come  into  vision,  and  no  others.  We  then  find  a  minus  cylin- 
der which,  with  its  axis  across  the  black  line,  puts  the  other 
focal  point  back  at  the  same  plane,  so  that  all  the  lines  are  seen 
equally.  This  cylinder  is  the  measure  and  correction  of  the 
astigmatism.    If  any  spherical  error  now  remain,  its  correction 

can  be  proceeded  with. 

f 
THE  STENOPAIC  SLIT. 

Another  subjective  test  is  by  means  of  the  stenopaic  slit — 
an  opaque  disk  with  a  single  straight  slit  in  it  through  w^hich 
the  light  will  enter  the  eye  along  one  meridian  only.  If  we 
revolve  this  slit  before  the  eye,  in  the  graduated  frame,  direct- 
ing the  patient  to  look  at  the  letter  chart,  we  shall  find  an 
angle  at  which  the  slit  will  give  the  best  vision,  and  one  at 
which  it  will  give  the  worst.     These  represent  the  two  chief 


58  ASTIGMATISM 

meridians.  All  we  now  have  to  do  is  to  find  a  spherical  lens, 
plus  or  minus,  which  will  make  vision  normal  through  the  slit 
at  each  oi  these  angles,  and  the  difference  of  dioptrism  between 
these  two  lenses  (calculated  algebraically)  will  be  the  measure 
of  the  astigmatism.    'I'he  balance  of  the  correcting  lens  power 


Stenopaio    Slit. 

will  represent  the  spherical  correction  to  be  combined  with 
the  cylinder. 

Thus,  if  we  find  that  with  the  slit  at  90  deg.  we  get  the  l)est 
vision,  but  it  requires  a  plus  2  1).  to  make  it  20/20;  and  at  180 
deg.  we  get  the  worst  vision,  requiring  plus  3  D.  to  make  it 
20/20;  the  patient's  astigmatism  is  the  difference  between  plus 
2  D.  and  plus  3  D..  i.  e.,  ])lus  1  D.  A  plus  1  D.  cylinder  with 
its  axis  at  SK)  deg.  (i.  e.,  across  the  most  hyperopic  meridian) 
will  correct  the  astigmatism,  making  both  meridians  alike.  The 
additional  plus  2  D.  sphere  will  correct  the  remaining  hyper- 
opia. This  example,  by-the-way.  is  a  case  of  compound 
hyperopic  astigmatism,   with   the  rule. 

Objective  methods  of  lindini;  and  estimating  astigmatism 
are  retinoscopy  and  ophthalmonu'lry.  Detailed  accounts  of 
tlte  ajjplication  of  these  two  instruments  to  astigmatism  will 
be  found  under  Retinoscopy  and  Ophthalmometry  rcsi»ecti\  ely. 

As  previously  stated,  a  small  percentage  of  astigmatisms 
are  due  to  non-spherical  curvature  of  the  crystalline  lens.  There 
is  no  objective  method  (tf  iletermining  lenticular  astigmatism, 
lience,  in  measuring  astigmatism  objectixe  ineiluids  sluuiKl 
always  be  checked  up  bx    subjective  tests. 

Astigmometer.     The  same  as  an  (  )phthaliiiometer.  i\.  v. 


ASYMMETRY  59 

Asymmetry.  A  lack  of  similarity,  in  size,  position,  etc.,  between 
the  two  things  that  are  being  compared — e.  g.,  of  the  sides 
of  the  face,  the  eyes,  etc.  No  human  being  has  symmetry  of 
any  part  of  the  two  sides  of  the  body ;  but  unless  it  is  noticeable 
we  do  not  call  it  asymmetry. 

Attollens.  Applied  to  a  muscle  which  raises  a  part.  Attollens 
oculi  is  the  superior  rectus  muscle  of  the  eye. 

Aura.  A  subjective  sensation  which  heralds  the  approach  of  a 
loss  of  consciousness,  as  in  epilepsy,  fainting,  etc.  Often  this 
aura  consists  in  a  visual  sensation,  such  as  a  scintillating  sco- 
toma, or  a  flash  of  light,  or  an  illusionary  image. 

Autophthalmoscopy.  The  art  of  viewing  one's  own  eye  through 
the  ophthalmoscope. 

Axanthopsia.     Yellow-blindness. 

Axis.  An  axis  is  an  imaginary  straight  line  drawn  through  a 
body,  or  a  system,  around  which  the  body  or  system  groups 
itself  symmetrically. 

The  axis  of  a  sphere  coincides  with  its  diameter,  and  may 
be  drawn  through  the  center  in  any  direction.  However,  for 
working  purposes,  when  once  an  axis  has  been  drawn  through 
a  sphere,  it  is  regarded  as  the  principal  axis,  and  all  other  lines 
drawn  through  its  center  are  regarded  as  secondary  axes.  In 
like  manner,  in  a  spherical  lens,  which  is  a  segment  of  a  sphere, 
the  principal  axis  passes  straight  through  the  center  of  its 
segmental  curvature  perpendicular  to  the  surface ;  all  other 
lines  passing  through  perpendicularly  to  its  surface  are  sec- 
ondary axes. 

The  principal  focal  point  of  a  spherical  refracting  or  reflect- 
ing system  is  always  situated  on  its  principal  axis. 

The  principal  axis  of  the  eye,  which  is  a  spherical  refracting 
system,  passes  through  the  center  of  the  cornea,  and  the 
geometric  center  of  the  eyeball,  to  the  geometric  center  of  the 
retina.  On  this  axis,  at  the  plane  of  the  retina,  is  the  principal 
focal  point  of  a  normal  eye.  Other  lines  which  pass  through 
the  cornea  perpendicularly  to  its  surface  are  secondary  axes. 

A  cylinder  has  two  axial  systems.  The  axis  which  passes 
through  the  center  of  the  circular  aspect  parallel  to  the  plane 


aspect  of  the  curved  side  is  called  the  cylinder  axis.  But  as  its 
refracting  power  lies  in  its  curvature,  it  is  ascribed  a  set  of 
axes  perpendicular  to  this  curvature,  passing  through  its  cir- 
cular center,  similar  to  those  of  a  sphere.  The  principal  axis 
passes  perpendicularly  through  the  center  of  its  curvature ;  all 
other  axes  passing  through  it  perpendicularly  to  its  curvature 
are  secondary  axes.  So  with  cylindrical  lenses,  which  are 
segments  of  cylinders. 

The  visual  axis  is  an  imaginary  line  drawn  straight  from  the 
yellow  spot  (moacula  lutea)  through  the  nodal  point  (q.  v.) 
to  the  object  looked  at.  It  will  be  seen  that  this  axis,  which 
is  the  line  of  attentive  vision,  does  not  coincide  with  the  prin- 
cipal axis  of  the  eye,  the  yellow  spot  being  a  little  to  the  tem- 
poral side  of  the  optical  center  of  the  retina. 

The  eye1)all  has  several  axes  of  rotation,  imaginary  straight 
lines  around  which  it  rotates,  according  to  the  muscle  or 
muscles  in  play  and  the  direction  of  their  action.  All  of  these 
axes  are  oblique,  owing  to  the  combined  action  of  the  oblique 
muscles  with  every  pair  of  recti. 

Axonometer.  An  instrument  for  rai)idly  determining  the  axis 
of  a  cylindrical  lens;  also  for  determining  the  axes  of  the  chief 
meridians  in  an  astigmatic  eye. 

Most  axonometers  of  the  first  kind  are  based  upon  the  fact 
that  when  a  straight  line  is  viewed  through  a  cylinder  other 
than  parallel  or  perpendicular  to  its  axis,  it  apparently  loses 
its  continuity  and  is  broken  into  two  lines  separated  laterall}" 
from  each  other.     (See  Lens.) 

Bacillar  Layer.     Tiie  rcjds  and  cones  of  tiie  retina. 

Band,  Astigmatic.  An  ai)parent  band  of  light  which  is  seen  to 
lie  across  the  pupil  of  an  astigmatic  eye.  under  retinoscopy, 
when  one  of  the  chief  meridians  is  niutrali/ed.  See 
Retinoscopy. 

Bands,  Spectrum.  .Spectra  of  gaseous  bodies,  consisting  of 
bright  bands  of  color. 

Base  Curve,  'ilu-  standaid  cur\  l-  which  oitticiaus  um-  in  j;rinding 
the  back  surface  of  loric  lenses.  The  other  surface  is  then 
ground  so  that,  in  combination  with  the  base  curve,  it  gi\es  the 


BAUM'S   OPHTHALMOSCOPE  61 

desired  compound  optical  effect.  Originally  there  were  three 
such  standard  base  curves,  3  D.,  6  D.,  and  9  D.,  but  nowadays 
opticians  vary  them  to  suit  their  facilities.     (See  Lens.) 

Baum's  Ophthalmoscope.  An  electrical  ophthalmoscope  invented 
by  the  German  ophthalmologist,  Baum,  in  which  the  light  is 
refracted  through  a  prism  instead  of  being  reflected  by  a  mirror. 

Bi-Astigmatism.  A  condition  of  the  eye  in  which  both  corneal 
and  lenticular  astigmatism  co-exist. 

Bi-Axial.  A  term  applied  to  those  crystals  which  perform  double 
refraction.     See  Diffraction. 

Bi-Concave.     Concave  on  both  sides.     (See  Lens.) 

Bi-Convex.     Convex  on  both  sides.     (See  Lens.) 

Bifocal.  Literally,  having  two  foci.  The  term  is  specifically 
applied  to  lenses  made  up  of  two  parts,  each  having  a  different 
refracting  power,  and  therefore  a  different  principal  focal  point ; 
one  part,  as  a  rule,  being  intended  for  distant  vision  and  the 
other  for  reading  and  close  work.  They  are  specially  applic- 
able in  presbyopia  (q.  v.). 

Bifocals  were  the  original  device  of  Benjamin  Franklin,  and 
there  have  been  innumerable  dift'erent  forms  devised  and  used 
from  Franklin's  time  to  the  present  day.  In  these  days,  how- 
ever, there  are  practically  but  three  forms  in  common  use : 

(1)  Cement  bifocals,  in  which  the  larger  refracting  power 
is  attained  by  cementing  a  wafer  of  greater  curvature  onto 
the  front  or  back  of  the  lens.  The  wafer  is  usually  either  cir- 
cular or  oval  in  shape,  and  placed  flush  with  the  lower  edge  of 
the  lens,  so  that  the  visual  axis  may  pass  through  it  when  the 
eyes  are  converged  and  lowered  for  reading. 

(2)  Onepiece  bifocals,  where  the  two  dift'erent  curvatures 
are  ground  on  the  same  piece  of  glass.  In  some  types  the  near 
vision  portion  practically  merges  into  the  distant  vision  portion 
or  main  lens;  in  others  the  near  vision  portion  or  segment  is 
depressed,  thus  forming  a  slight  abrupt  ridge  or  shoulder 
separating  the  two  portions  of  the  lens. 

(3)  Fused  bifocals,  in  which  the  higher-power  part  is  fur- 
nished by  a  fused  insert  of  a  denser  quality  of  glass.  Usually 
flint  glass  is  fused  into  crown  glass.     The  different  melting 


6J 


BINOCULAR 


point  of  the  two  glasses  permits  tliis  to  be  done;  the  greater 
optical  density  of  the  flint  glass  allows  a  higher  refracting 
power  to  be  obtained  with  proportionately  less  increase  of 
curvature:  and  the  line  of  junction  between  the  two  parts  is 
practically  invisible.  These  lenses  are  sometimes  called  "In- 
\isible''  Bifocals. 

Binocular.  The  word  is  applied  to  anything  that  relates  to  the 
simultaneous  use  of  both  eyes  in  the  act  of  vision.  In  regard 
to  the  eyes  themselves,  it  signifies  any  function  in  which  both 
eyes  partake  together,  as  binocular  accommodation,  binocular 
squint,  etc.  In  regard  to  instruments,  it  signifies  that  the  in- 
strument is  to  be  used  by  both  eyes  together,  as  binocular 
telescope,   etc. 

All  functions  of  the  c\e  are  performed  more  \igorously  in 
binocular  than  in  monocular  vision. 

Binocular  Vision.  The  single  eye  surveys  a  visual  field  which 
iias  a  range  of  approximately  180  degrees  horizt)ntall\  and  1.^0 
degrees  vertically.  Inner  portions  of  these  two  \  isual  held> 
overlap,  when  both  eyes  are  used  together,  forming  a  cone, 
with  the  nose  as  a  vertex,  in  which  lioth  eyes  see  simultaneous- 
ly and  coincidcntly.     This  is  the  binocular  \  isual  field. 

CORRESPONDING   AND   DISPARATE  POINTS   OF  THE 
RETINA. 

In  order  to  obtain  a  single  image  of  an  object  in  the  binocular 
visual  field,  if  the  object  is  pictured  on  a  certain  position  on 
one  retina  it  must  be  i)ictured  on  a  certain  definite  and  sym- 
metrical i)osition  on  the  other  retina.  Portions  of  the  two 
retinae  which  work  together  in  this  \\a\  are  known  a^  cor- 
responding, or  identical.  ])oints;  porti()ns  which  ha\e  not  this 
coincidence  are  called  disparate  jxtints.  bVom  the  fact  that 
we  see  objects  singly  upon  which  we  fix.  it  is  apparent  that  the 
two  yellow  spots  are  corresponding  retinal  points. 

It  is  evident  that  as  long  as  the  head  is  held  in  one  position 
the  portion  (»f  space  represent ini;  the  binocular  visual  field  re- 
mains the  same;  and  that  as  the  head  and  eyes  are  moved,  the 
size  and  position  and  contour  of  this  tield  change.  \\  ithin 
the  binocular  lield-  and  oulv  within  ili.it  lieid  -there  are  cer- 
tain  j)oints   in    space   wlu>se   image  points    will    fall    upon    cor- 


BINOCULAR   VISION 


63 


responding'  points  of  the  two  retinae,  and  be  seen  singly.  The 
totality  of  these  points  is  termed  the  horopter. 

It  is  possible  to  determine,  mathematically,  for  every  posi- 
tion of  the  eyes,  just  what  points  in  space  will  fall  upon  cor- 
responding points  of  the  retinae ;  that  is  to  say,  the  extent  and 
contour  of  the  horopter.  The  horopter  is  a  curve  of  the  third 
degree.  Three  points  through  which  it  must  pass  are  pre- 
determined ;  they  are  the  point  of  fixation  and  the  nodal  points 
of  the  two  eyes.  The  latter  points,  and  adjacent  parts,  are, 
of  course,  not  pictured  on  the  retina  at  all ;  this  portion  of  the 
horopter  is  a  purely  mathematical  quantity. 

When  the  visual  axes  are  parallel  and  symmetrical  in  rela- 
tion to  the  median  line,  the  fixation  point  is  at  infinity.  If  the 
middle  cross-sections  of  the  visual  field  are  horizontal,  the 
transversal  planes  which  cut  the  retina  in  corresponding  lines 
are  coincident,  and  the  entire  binocular  visual  space  is  the 
horizontal  horopter.  If  the  cross-sections  make  an  angle  with 
each  other,  the  transversal  planes  intersect  at  the  median  line. 
and  this  plane  is  the  horizontal  horopter.  If  the  middle  longi- 
tudinal sections  of  the  field  are  vertical,  the  corresponding 
longitudinal  planes  intersect  at  mfinity,  and  this  is  the  vertical 
horopter.  The  point-horopter  is  the  intersection  of  these  two 
line-horopters. 

When  the  visual  axes  are  symmetrical,  but  not  parallel,  the 
fixation  point  is  in  the  median  line  at  a  finite  distance.  The 
visual  plane  is  the  horizontal  horopter.  The  vertical  horopter 
is  a  cylinder  perpendicular  to  the  visual  plane  whose  section 
by  the  plane  is  a  circle  passing  through  the  point  of  fixation 
and  the  nodal  points  of  the  eyes  (Muller's  circle). 

When  the  point  of  fixation  is  in  the  horizontal  plane  and 
the  visual  axes  are  symmetrical,  the  vertical  horopter  is  a 
hyperboloid  whose  section  with  the  median  plane  is  Muller's 
circle.  The  horizontal  horopter  consists  of  the  visual  plane 
and  a  plane  perpendicular  to  it  passing  through  the  intersec- 
tion of  Muller's  circle  with  the  median  plane  and  through  one 
end  of  the  diameter  of  this  circle  which  goes  through  the  point 
of  fixation.  The  point-horopter  is  Muller's  circle  and  a  straight 
line  inclined  to  the  visual  plane  which  passes  through  the  in- 
tersection point  just  described. 


64  BINOCULAR   VISION 

STEREOSCOPIC  VISION. 

Visual  sensations  are  referred  by  projection  to  space  of  three 
dimensions,  namely,  length,  breadth  and  depth.  Perceptions 
of  length  and  breadth  are  jjroperties  of  monocular  vision,  and 
pertain  to  any  and  all  portions  of  the  visual  field.  Perception 
of  depth,  which  is  less  perfectly  developed,  is  possible  only 
with  binocular  vision,  therefore  pertaining  only  to  the  binocu- 
lar visual  field,  and  is  an  optical  functicjn  of  the  horopter. 

No  doubt  much  of  our  idea  of  depth  arises  from  elements  of 
experience,  which,  of  course,  is  not  a  direct  sensation,  but  an 
inference  from  other  sensations.  The  comparison  of  the  ap- 
parent size  of  various  objects,  for  instance,  enables  us  to  form 
a  judgment  as  to  their  respective  distances.  Simple  geometrical 
forms  with  which  we  are  \ery  familiar  produce  an  impression 
of  solidity.  The  distri])ution  of  light  and  shade  in  the  field  of 
view  is  another  such  factor — not  alone  the  shadows  cast  by 
objects  themselves,  but  the  degrees  of  illumination  according 
as  they  are  turned  toward  or  away  from  the  source  of  light. 
And  finally,  so-called  aerial  perspective  is  an  important  ele- 
ment, i.  e.,  dimmed  or  veiled  ap])earance  of  objects,  or  their 
parts,  in  proportion  to  the  depth  of  atmosphere  through  which 
we  view  them. 

Aside  from  these  experiential  factors,  however,  the  eyes 
possess  the  faculty  of  percei\ing  depth  as  a  direct  sensation, 
which,  as  stated,  jicrlains  wholly  to  l)in(icular  \ision.  The 
muscle  sense,  as  exercised  in  accommodation  and  ct)n\ergence. 
is  generally  accepted  as  forming  one  of  the  component  ele- 
ments of  this  sensation.  It  is  (|uestionable.  however,  wlu'ther 
a  judgment  derived  from  the  muscle  sense  can  properly  be 
regarded  as  part  of  an  immediate  sensation  :  and.  besides,  it 
plays  a  very  small  and  iminiiiortaiit  pail  in  (Ifptli  |>i'rceplion. 
being  necessarily  restricted  to  the  relati\ely  small  range  of 
accommodation  and  conxergence.  'The  fact  that  we  perceive 
depth  ill  :iii  object  inider  sudiU-ii  and  iiioiiu'iitary  illumination; 
the  further  fact  that  facsimiles  make  eipial  impressions  of 
scilidity;  and  the  still  further  fact  that  we  are  able  to  perceive 
siijid.irity  tar  beyond  the  laiigi'  of  acidiiiiiii'datii  m  :md  con- 
vergence; ;ill  ptiiiit   til  the  itisigiiilicaiue  tif  this  element. 


BINOCULAR  VISION  65 

By  far  the  most  important  factor  in  the  sensation  of  depth 
is  the  difference  in  the  images  made  by  a  solid  object  on  the 
two  retinae,  respectively,  and  the  difference  in  the  relations  of 
the  horopter  with  the  disparate  object-points,  due  to  the  lateral 
separation  of  the  two  eyes  by  the  interpupillary  distance ;  that 
is  to  say,  to  the  parallax  of  the  two  eyes.  This  perception  of 
depth  is  known  as  sterescopicf  vision.  Stated  mathematically, 
the  stereoscopic  parallax  depends  upon  the  distance  between  a 
point  of  the  object  and  the  vertical  plane  through  the  nodal 
points  of  the  eye,  and  is  inversely  proportional  to  this  distance. 
This  difference  arises  in  the  moving  of  images  in  the  direction 
of  the  line  connecting  the  eyes ;  hence  the  use  of  the  word 
"vertical"  in  the  definition  just  given.  It  is  impossible  to  per- 
ceive the  difference  in  depth  of  a  set  of  telegraph  wires  strung 
horizontally ;  but  if  the  same  group  of  wires  are  strung  verti- 
cally, the  dift'erence  of  depth  is  easily  recognized. 

Stereoscopic  vision  is  both  direct  and  indirect.  The  direct 
exercise  of  the  sense  depends  upon  comparison  of  disparate 
points  in  the  visual  field  with  the  fixation  point;  the  indirect, 
upon  comparison  with  one  disparate  point  with  another.  In- 
direct stereoscopic  vision  is  exceedingly  important  in  daily  ex- 
perience, as  it  serves  to  protect  us  against  danger  from  ap- 
proaching objects  of  danger  outside  the  range  of  the  horopter. 

The  differences  in  depth  which  are  stereoscopically  percept- 
ible vary  directly  with  the  square  of  the  mean  distance  of  the 
points.  Helmholz  found  that  a  difference  of  1  minute  of  arc  is 
sufficient  to  be  perceived.  (It  will  be  observed  that  this  cor- 
responds to  the  minimum  visual  angle.)  The  pupillary  width 
in  an  average  individual  is  68  mm.  Therefore,  according  to  the 
form.ula, 

68 

=  240  meters 

sine  1' 
240  meters   is   the   greatest   distance    for   stereoscopic    vision, 
and  is  known  as  the  stereoscopic  radius. 

A  1  minute  angle  corresponds  to  .10  mm.  in  lengths.  There- 
fore, pictures  which  differ  .10  mm.  manifest  the  difference  if 
looked  at  stereoscopically.  This  fact  is  utilized  for  the  detec- 
tion of  counterfeit  bank  notes  and  bills. 


66 


BINOCULARS 


There  are  two  ways  of  increasing  the  limit  of  stereoscopic 
vision,  viz.,  (1)  By  increasing  the  keenness  and  range  of  vision, 
as  in  the  case  of  the  telescope  and  microscope,  and  (2)  By  in- 
creasing the  inter-pupillary  distance,  by  means  of  reflecting 
mirrors.  All  instruments  designed  for  this  purpose,  and  known 
as  stereoscopes,  are  based  upon  these  principles.  (See  Stereo- 
scope.) 

STRUGGLE  OF  THE  VISUAL  FIELDS. 

When  the  two  visual  fields  are  represented  by  images  wholly 
dissimilar  lo  each  other,  it  is  impossible  to  fuse  them,  and  one 
or  other  of  the  two  fields  predominates  over  the  other;  usually 
they  take  the  predominance  alternately.  This  condition,  which 
in  its  extreme  form,  is  known  as  the  antagonism  of  the  visual 
fields,  obtains  to  a  more  or  less  degree  in  all  binocular  vision, 
constituting  what  is  known  as  the  struggle,  or  rivalry,  of  the 
visual  fields. 

Binoculars.  A  telescope  with  two  barrels  or  lens  systems,  one 
for  each  eye;  sometimes  called  "field  glasses,"  but  the  term  is 
applied  by  the  optical  trade  generally  to  field  glasses  in  which 
reflecting  prisms  are  employed. 


IJiiuHuhir.s. 


Bi-Orbital.     Kclnting  to  both  orbits. 

Blenorrhea.     A  profuse  How  of  pus  from  the  eye.      1  echnically 
the  W(M(1  is  generally  used  to  (Kiujte  gonorrheal  conjuncti\  ills. 

Blepharism.     A  tendency  to  winking.     May  be  the  result  of  eye- 
strain, due  to  errors  of  refraction. 

Blepharitis.      lnllammatit)n   of  the  eyelids.     Often   the   rrsult   of 
eye-strain,  due  to  refractive  errors. 

Blepharospasm.     Twitching  of  the  eyelids,  often  ilue  to  eyestrain 
from  uMconecteil  errors  (»f  refr;ielion. 


Blepharoplegia. 


f  th< 


arausis  ot   the  evelids. 


BLEPHAROPTOSIS  67 

Blepharoptosis.     Dropping  of  the  eyelid. 

Blepharostat.     An  instrument  for  holding  the  eyelids  apart. 

Blepharostenosis.     Abnormal  narrowing  of  the  palpebral  slit. 

Blepharosynechia.     Adhesion  of  the  eyelids  to  each  other. 

Blepharotomy.     A  cutting  operation  on  the  eyelids. 

Blindness.  The  word  blindness  is  used  to  describe,  in  general, 
an  inability  to  see,  from  whatever  cause.  Blindness  may  be 
partial  or  complete,  relative  (amblyopia)  or  absolute  (amauro- 
sis).    Special  forms  of  blindness  are: 

Color  Blindness.  Inability  to  distinguish  all  or  some  of  the 
colors.    (See  Color  Blindness.) 

Day  Blindness.  A  condition  in  which  the  patient  sees  badly 
in  the  day-time,  better  in  the  evening. 

Letter  Blindness.  Inability  to  recognize  letters  either  all  the 
letters  of  the  alphabet,  or  certain  of  them.  It  is  a  form  of 
mind-blindness. 

Night  Blindness.  A  condition  in  which  the  patient  sees 
poorly  at  night,  better  in  the  daylight. 

Number  Blindness.  Inability  to  recognize  numerals,  all  or 
certain  of  them.     It  is  a  form  of  mind-blindness. 

Mind  Blmdness.  Inability  to  give  association  or  meaning  to 
what  one  sees.  Due  to  the  disease  of  the  great  association 
areas  of  the  brain. 

Object  Blindness.     Same  as  mind-blindness. 

Snow  Blmdness.  Blindness  from  retinal  exhaustion,  due  to 
exposure  to  the  glare  of  light  on  snow. 

Unilateral  Blindness.     Blindness  of  one  eye  only. 

Word  Blindness.  Inability  to  recognize  words,  all  of  them 
or  certain  words.     It  is  a  form  of  mind-blindness. 

Blind  Spot.  The  circular  spot  on  the  retina  formed  by  the  en- 
trance of  the  optic  nerve.  At  this  spot  the  retina  is  insensible 
to  light  stimulation ;  and  light  waves  or  images  that  fall  on  this 
area  are  not  perceived.  Advantage  is  taken  of  this  fact  to  focus 
light  on  the  blind  spot  when  examining  the  interior  of  the  eye 
with  the  ophthalmoscope. 
(See  Optic  Disc.) 


68 


BLOOD  PRESSURE 


Blood  Pressure.  The  word  "pressure''  is  hardly  the  correct  term 
to  apply  to  the  phenomenon  which  it  is  here  intended  to  define. 
Tension  would  be  a  better  word.  Pressure  is  a  one-sided  exer- 
cise of  power,  which  may  be  directed  in  any  degree  against 
a  yielding  body,  offering  no  resistance,  without  the  production 
of  any  tension  at  all.  On  the  other  hand,  half  the  same  amount 
of  pressure,  divided  equally  between  two  bodies  and  exerted 
against  each  other,  will  develop  a  large  degree  of  tension.  In 
fact,  the  tension  thus  developed  is  a  physical  and  a  mathemati- 
cal product  of  the  two  opposing  pressures;  regarding  one  of 
thcni  as  reiiresenting  force,  and  the  other  resistance,  then 

p  =  fr 
where  p  stands  for  tension,  f  for  force,  and  r  fur  resistance. 
Thus,  in  exemplification  of  the  statement  made  above,  if  we 
ccjnsider  a  force  of  20  lbs.  acting  against  a  body  whose  resist- 
ance is  0,  then  the  tension  developed  is  equal  to  20  X  0  =  0. 
If,  on  the  other  hand,  a  force  of  10  lbs.  be  directed  against  a 
l)ody  offering  10  lb.  resistance,  then  the  tension  developed  will 
be  10  X  10  =  100  lbs. 

However,  as  the  term  Bloucl  Pressure  has  become  general, 
we  shall  adhere  to  it,  with  the  understanding  that  it  is  used  in 
the  sense  of  inLer-ol)jective  tension.  In  this  sense.  Blood  Pres- 
sure is  the  product  of  the  force  of  the  contraction  of  the  heart 
muscle  and  the  resistance  of  the  blood  vessels,  operating  upon 
and  manifested  through  the  lluid  blood.  I'or  a  thorough  un- 
derstanding; of  the  subject,  of  course,  it  is  necessary  to  have  a 
comj^reliensixe  knowledge  of  the  whole  anatomy  and  physiol- 
ogy, not  only  of  the  circulatory  system,  but  also  of  the  entire 
body,  since  there  is  no  part  or  function  but  plays  a  part  in  both 
the  production  and  the  conse(|uences  of  blood  ]>ressure.  We 
will,  however,  content  ourselves  with  giving  here  a  brief  sum- 
mary of  the  subject,  with  special  refen-noe  t»)  its  niechanics 
.ind  its  rlinic.'d  as])i'Cts. 

MI'XHANICS    01<    BLOOD   PRESSURE. 

The  blood,  as  e\i'ryonc'  understands,  lies  within  :i  closed  sys- 
tem of  vessels,  in  which  it  is  kept  mo\ing  round  and  round  by 
means  (»f  a  i)ump,  in  much  the  same  way  as  the  water  in  a  hot- 
water  healing  system.  It  is  trur  tiiat  this  system  of  l)lood 
\  essels  is  not  absolutely  w.iter  tiL;lil  ;  tli.it  its  w.ills  are  perme- 


BLOOD  PRESSURE  69 

able,  so  that  some  of  the  fluid  and  a  great  deal  of  the  gaseous 
content  of  the  fluid,  is  continually  passing  in  and  out  of  the 
vessels ;  but,  as  the  outgoing  and  incoming  balance  is  main- 
tained virtually  constant,  it  may  be  regarded,  so  far  as  its  me- 
chanics are  concerned,  as  a  water-tight  system. 

The  pump,  in  this  case,  is  the  heart,  a  large,  powerful,  hol- 
low muscle,  filled  with  blood,  whose  vigorous  contraction 
throws  a  large  volume  of  blood  into  the  vessels,  and  forces  the 
blood  stream  along  them,  and  whose  relaxation  causing  a  dila- 
tation of  the  hollow  chamber,  and  forming  a  vacuum,  draws 
blood  into  itself,  by  suction,  ready  for  the  next  contraction. 
These  contractions  and  dilatations,  technically  known  as  the 
systole  and  diastole  of  the  heart,  respectively,  alternate  with 
each  other  rhythmically,  with  a  frequency  of  from  60  to  80 
cycles  per  minute.  As  a  matter  of  fact,  the  heart  is  a  double 
organ,  having  two  separate  chambers,  pumping  and  exhaust- 
ing Iwo  separate  systems  of  vessels.  One,  the  shorter  of  the 
two,  carries  vitiated  (venous)  blood  to  the  lungs  and  brings  it 
back  to  the  heart  aerated,  and  is  operated  by  the  right  side  of 
the  heart;  the  other,  the  longer  system,  carries  aerated  (arter- 
ial) blood  through  the  body  at  large,  and  is  operated  by  the 
left  side  of  the  heart.  It  is  of  the  latter  system  that  we  shall 
speak  here,  although  what  is  true  of  the  one  holds  substantially 
good  for  the  other. 

The  lay-out  and  character  of  the  vessels  through  which  the 
blood  is  forced  form  a  most  important  part  of  the  mechanism 
of  blood  pressure.  The  heart  empties  itself  into  one  large 
artery  (the  aorta),  which  divides  into  two  smaller  arteries, 
whose  total  wall-area,  however,  is  greater  than  that  of  the 
aorta;  these  two  divide  similarly  into  four;  the  four  into  eight; 
and  so  on,  until  all  rather  abruptly  subdivide  into  thousands  of 
tiny  capillaries,  whose  total  wall-area  is  some  800  times  that 
of  the  aorta.  On  the  other  side  of  this  capillary  system  the 
vessels  just  as  abruptly  recombine  into  an  even  number  of 
small  vessels,  called  veins,  which  then  successively  reunite, 
in  a  progression  the  reverse  of  the  subdivision  of  the  arteries, 
i.  e.  each  time  into  half  the  number  of  vessels  with  a  less 
total  area,  until  at  last  two  large  veins  pour  the  blood  back 


70 


BLOOD  PRESSURE 


into  the  right  chamber  of  the  heart.     These  veins  are  called 
the  superior  and  inferior  venae  cavae. 

The  arteries  are  furnished  with  elastic  tissue  which  gives 
them  elastic  properties ;  i.  e.  they  are  distensible  under  force, 
but  their  elastic  recoil  tends  to  restore  them  to  their  original 
size.  They  are  also  furnished  with  circular  muscles,  forming 
part  of  their  walls,  (vaso-motor  muscles),  by  whose  contrac- 
tion and  relaxation  the  vessels  can  be  contrnctt'd  and  dilated, 


T>'co  Sphygmoinaiiomi'tcr.  Mi.    Ii<;u'l)l<,Ts   e'abiii't    Si/-    Spin  ;;n\Mmanuiiut<'r. 

respecti\'ely.  The  larger  arteries  are  ])riniipally  rich  in  elastic 
tissue;  in  the  smaller  arteries  the  muscular  element  predomin- 
ates. The  cai)illaries  are  neither  elastic  nor  muscular.  I'he 
veins  arc  both,  lo  a  very  slight  degree,  but  \s  compared  with 
the  arteries  we  may  regard  them  as  almost  inelastic  and  non- 
muscular. 

The  force  of  the  heart-beat  expends  itself  in  maintaining  the 
pressure  in  the  arteries;  it  is  lost  li\  the  time  the  blood  enters 
the  capillaries.     When  the  blood  emerges  from  the  capillaries 


BLOOD  PRESSURE  71 

into  the  veins,  some  slight  impetus  is  furnished  it  in  the  suc- 
tion referred  to  above,  due  to  the  vacuum  made  by  the  dilata- 
tion of  the  heart  chamber  in  diastole;  but  chiefly  the  blood  is 
kept  moving  by  the  difference  of  pressure  in  the  arteries  and 
in  the  veins,  and  by  means  of  the  valves,  all  closing  backward, 
with  which  the  heart  and  the  veins  are  supplied. 

We  have,  then  as  factors  in  the  resistance  offered  by  the 
vessels  to  the  flow  of  blood,  the  following : 

(1)  Friction  between  the  blood-stream  and  the  vessel  walls, 

(2)  The  elastic  recoil  of  the  artery  walls, 

(3)  The  contraction  of  the  vaso-motor  muscles, 

The  net  resultant  of  the  product  of  these  factors  on  the  one 
hand,  and  the  force  of  the  heart  beat  on  the  other  hand,  con- 
stitutes the  blood  pressure.  To  which  may  be  added,  as  a  vari- 
ant factor,  gravity,  or  the  weight  of  the  column  of  blood  above 
the  point  where  the  measurement  is  made,  depending  upon 
posture,  etc. 

Naturally,  the  blood  pressure  is  not  the  same  at  various 
points  in  the  system.  Leaving  out  of  consideration,  for  the 
moment,  the  muscular  element,  it  is  evident  that  the  farther 
away  from  the  heart  we  get,  the  less  becomes  the  force  of  the 
heart-beat  and  the  greater  becomes  the  resistance — especially 
that  offered  by  friction,  since  the  total  wall-area  is  progres- 
sively increasing  along  the  arterial  system.  The  blood  pres- 
sure is  greatest  in  the  aorta,  for  there  the  force  of  the  heart  is 
at  its  maximum  and  the  average  of  resistance  is  pretty  high, 
and  the  product  of  the  two  is  greater  than  at  any  other  point. 
In  the  capillaries  it  is  exceedingly  low,  for  there  the  force  of 
the  heart  is  lost,  and  the  resistance  is  very  high.  In  the  veins 
it  rises  again  somewhat,  for  the  vacuum  suction  increases  the 
force  a  little  and  resistance  is  diminished.  At  the  venae  cavae, 
however,  where  the  veins  empty  into  the  heart,  there  is  neither 
force  nor  resistance,  hence  pressure  at  this  point  is  zero. 

All  of  this,  however,  is  reckoning  without  the  vaso-motors, 
which  play  a  larger  part  in  variations  of  blood  pressure  than 
any  other  single  factor.  These  muscles  are  under  the  nerve 
control  of  the  sympathetics.  Normally,  their  office  is  to  equal- 
ize the  pressure  as  between  two  or  more  areas,  so  as  to  main- 
tain a  general  average  of  pressure  throughout  the  body.   Thus, 


/I 


BLOOD  PRESSURE 

(luring  digestion,  the  splanchnic  vessels  are  dilated  and  filled 
with  blood  at  the  expense  of  the  vessels  of  the  skin  and  mus- 
cles. The  latter  vessels  being  partially  emptied,  the  pressure 
in  them  would  fall ;  but  the  vaso-motors  automatically  contract, 
lessening  their  diameter,  and  maintaining  normal  pressure.  In 
neurotic  people,  and  in  certain  nervous  diseases,  the  vaso- 
motors behave  very  erratically,  causing  all  sorts  of  pressure 
disturbances — of  which  more  later. 

SYSTOLIC  AND   DIASTOLIC  PRESSURE. 

As  already  explained,  the  active  cause  of  the  pressure  ii)  the 
arteries  is  the  contraction  of  the  heart  muscle,  forcing  the 
blood  into  and  along  the  arteries  against  the  resistance  of  the 
vessels.  Naturally,  this  pressure  varies  at  different  points  of 
time  in  the  heart  cycle.  It  is  highest  at  the  moment  of  maxi- 
mal contraction  of  the  heart,  i.  e.  during  systole.  During  dia- 
stole, when  the  muscle  is  resting  and  the  heart  dilating,  it  falls, 
its  lowest  point  being  reached  at  the  end  of  the  heart's  resting 
l)eriod  immediately  preceding  another  contraction.  During 
this  period  the  pressure  relation  between  heart  and  vessels  al- 
most reaches  stable  equilibrium  ;  it  ne\er  (piite  reaches  this 
state,  because  the  next  heart-beat  intervenes  too  quickly  ;  but 
for  all  practical  purposes  the  pressure  condition  during  diastole 
nia\'  be  regarded  as  static. 

The  maximal  degree  of  pressure  attained  during  systole  is 
tcrnied  the  s\  slolic  or  dynamic  pressure;  the  lowest  degree 
iluring  diastole,  the  diasttilic  or  static  pressure;  the  difference 
between  the  two  necessarily  represents  the  added  pressure  due 
to  the  contraction  of  the  heart,  and  is  known  as  the  jiulse 
jjressure. 

METHODS  OF  MEASURING  BLOOD  PRESSURE. 

The  first  crude  method  of  (hnionstrating  and  measuring 
blo(»(l  ])ressure,  (employed  onl\  on  lower  animals),  was  to 
lliruhl  into  the  \essel  a  cannul.i  attached  to  a  graduated  glass 
tube,  and  note  the  height  to  which  the  blood  rose  in  the  tube 
during  syst(jls  and  diastole.  Modeiu  methods  and  instruments 
depend  upon  the  principle  th.tt  it'  \vi'  compress  an  artery  until 
its  wall  collapses,  the  picssme  can  then  bi'  di\erte<l  to,  and 
taken   up   by,    the   in.slrunicnt    with    which    the   compression    is 


BLOOD  PRESSURE  73 

made,  which  can  be  constructed  so  as  to  register  it  in  some 
unit  of  measurement. 

The  instrument  thus  used  is  called  a  sphygmomanometer. 
There  are  several  makes  on  the  market,  but  all  are  essentially 
upon  the  same  principle.  The  principal  difference  is  in  the 
method  of  registering  the  pressure,  of  which  there  are  two 
general  types.  In  one  form  of  instrument  the  pressure  is  meas- 
ured by  the  height  of  a  column  of  mercury — this  is.  perhaps, 
the  most  accurate,  and  is  the  form  generally  used  in  laboratory 
work,  where  great  precision  is  important.  The  other  type 
registers  the  pressure  through  a  coil  spring,  which  revolves  an 
index  pointer  on  a  graduated  dial ;  this  type  is  accurate  enough 
for  clinical  purposes,  and  much  more  portable  and  convenient. 

The  compression  is  usually  made  by  wrapping  around  the 
arm  a  large  silk  cuff",  which  is  an  air-tight  pneumatic  sac,  and 
pumping  air  into  it  until  the  pulsations  on  the  distal  side  of 
the  cuff  completely  disappear.  A  tvibe  from  this  sac  leads  to 
the  registering  device,  and  the  air-pressure  in  the  sac  (which  is 
the  equivalent  of  the  blood-pressure  it  collapses)  is  recorded. 
\Mth  some  instrvmients,  instead  of  the  cuff,  a  small  piece  of 
hard  substance  (rubber)  is  strapped  down  on  the  arm,  over  the 
artery,  this  compressor  communicating  with  an  air-chamber  or 
direct  with  the  mercury.  \Ye  shall  confine  ourselves  to  the 
former  type  of  sphygmomanometer  in  describing  the  technique 
of  the  procedure. 

TECHNIQUE. 

Blood  pressure — by  which  is  meant  arterial  pressure,  the 
only  phase  that  concerns  us — is  practically  always  measured 
in  the  brachial  artery  of  the  left  arm,  because  of  its  conven- 
ience. In  general,  the  patient  should  be  lying  down,  although 
it  is  sometimes  desirable  to  make  the  test  both  standing  and 
lying.  In  any  case,  successive  tests  should  be  made  with  con- 
ditions the  same. 

A  sphygmomanometer  of  the  type  we  are  considering  has  a 
compression  cuff,  a  piston  or  rubber  ball  for  inflating  it,  a  lever 
valve  for  deflation,  and  a  dial,  with  an  index  needle,  graduated 
into  equivalents  of  millimeters  of  mercury. 

Wrap  the  cuff'  evenly  and  tightly  around  the  upper  arm,  the 
first  lap  of  silk  being  placed  over  the  inner  aspect  of  the  arm, 


74 


BLOOD  PRESSURE 


and  tuck  the  final  lap  between  the  cuff  and  the  arm.  Place  the 
fingers  of  one  hand  on  the  radial  artery  at  the  wrist,  and  with 
the  other  inflate  the  cuff  until  all  pulsation  at  the  wrist  dis- 
appears and  there  is  no  movement  of  the  index  pointer  on  the 
dial,  indicating  that  the  arterial  pressure  has  collapsed  and 
there  is  a  good  margin  of  air  pressure  over. 

Two  methods  of  procedure  are  now  open,  the  palpation 
method  and  the  auscultation  method. 

Palpation  Method.  \\'ith  the  fingers  still  on  the  radial  ar- 
tery, the  air  is  gradually  let  out  of  the  cuff  by  means  of  the 
valve,  meanwhile  watching  the  fall  of  the  needle  on  the  dial. 
As  it  falls  it  begins  to  vibrate  ;  the  first  vibrations,  however, 
are  due  to  the  shaking  of  the  cuff  by  the  pulsations  above  it, 
and  have  no  significance.  In  fact,  the  vibrations  need  not  be 
regarded  at  all  at  this  time.  Presently  the  pulse  will  be  just 
faintly  felt  at  the  wrist.  Naturally,  it  first  comes  through  at 
the  point  of  maximal  pressure,  i.  e.  systolic  pressure.  The  read- 
ing on  the  dial  at  this  point,  then,  represents  systolic  pressure. 

Continue  to  let  the  air  slowly  out  of  the  cuff',  carefully  watch- 
ing the  dial.  As  the  full  pulse  begins  to  come  through,  the 
needle  will  be  seen  rather  abruptly  to  make  very  wide  excur- 
sions to  and  fro  in  its  \  ibrations.  The  point  of  maximum  ex- 
cursions, and  the  reading  on  the  dial  at  this  point,  represent 
diastolic  pressure.  The  palpation  method,  however,  is  subject 
to  so  many  errors  that  it  has  been  almost  entirely  supplanted 
by  the  auscultation  method. 

Auscultation  Method.  For  this  jiroccdure  a  stethescope  with 
a  small  flat  transmitter  is  recjuired.  Neither  the  pulse  nor  the 
character  of  the  needle  vibrations  need  be  regarded  here,  ex- 
cept as  confirmatory  data,  since  the  reading  of  the  pressure  de- 
]jcnds  wholly  upon  the  sounds  heard  thrt)ugli  the  stethescope. 

Place  the  transmitter  of  the  stethescope  firmly  over  the  ar- 
tery just  above  or  just  below  the  inner  bend  of  the  elbow,  with 
the  ear-pieces  in  your  ears.  Inflate  the  cufl',  as  described  abt)ve 
until  absolutely  no  sounds  are  heard  in  tlu-  ^tctlu-scope.  Slow- 
ly allow  the  air  to  escajjc.  The  first  sounds  to  be  heard  are 
dull,  indistinct,  irregular  beatings,  which  may  be  disreg.irded. 
as  they  arc  dui-  tu  tlu-  \iliiatii>ns  of  the  cutV.  C'diilinuiiig  tn 
ileflate   slowly,    llie    sounds    suddenly    change    to    sharp,    loud. 


BLOOD  PRESSURE  75 

rhythmical  clicks.  The  point  where  this  change  takes  place 
denotes  the  point  of  systolic  pressure,  and  should  be  read  on 
the  dial.  Again  continue  deflating,  until  the  sharp  clicking 
sounds  abruptly  change  to  soft  blowing  sounds.  The  place 
where  this  first  occurs,  and  the  reading  on  the  dial,  represent 
the  diastolic  pressure. 

We  have,  then,  four  phases  in  this  process,  namely: 

(1)  The  phase  of  silence,  signifying  arterial  collapse, 

(2)  The  phase  of  indistinct,  irregular  sounds,  having  no 
clinical  significance, 

(3)  The  phase  of  sharp,  clicking  sounds,  denoting  systolic 
pressure, 

(4)  The  phase  of  soft  blowing  sounds,  denoting  diastolic 
pressure. 

NORMAL  PRESSURE. 

The  normal  blood  pressure  varies  within  the  range  of  sev- 
eral millimeters,  both  systolic  and  diastolic,  in  different  indi- 
viduals so  that  only  an  approximate  average  can  be  given.  It 
also  varies  with  age,  due  to  the  progressive  loss  of  elasticity  in 
the  artery  walls,  and  the  increase  of  heart  muscle  which  com- 
pensates for  the  rise  in  resistance. 

In  a  young  male  adult  of  20  years  the  average  normal 
diastolic  pressure  is  about  70  mm.,  systolic  100  mm.  From 
that  time  on  the  diastolic  pressure  rises  approximately  0.5  mm. 
for  each  year  of  life,  and  the  systolic  about  1  to  1.25  mm.  for 
each  year.  This,  however,  as  stated,  is  subject  to  considerable 
variation  within  the  normal.  Females  usually  have  slightly 
lower  pressure  than  males. 

It  will  be  seen  that  the  difference  between  diastolic  and  sys- 
tolic pressure  (representing  the  pulse  pressure)  widens  each 
year,  as  the  systolic  pressure  increases  by  a  greater  ratio  than 
the  diastolic.  The  heart  is  obliged  to  put  on  an  excess  of 
muscle  power  in  order  to  overcome  the  increased  resistance  of 
the  vessels. 

PHYSIOLOGIC  VARIATIONS. 

Everything  else  being  equal,  blood  pressure  is  raised  by  any- 
thing which  increases  either  the  force  of  the  heart  or  the  re- 
sistance in  the  vessels,  or  both.     It  is  lowered  by  anything 


76  BLOOD  PRESSURE 

which  decreases  either  or  both  of  these  factors.  Practically 
all  physiologic  elevations  of  pressure  are  brought  about  by  in- 
creased heart  action,  either  through  muscular  exercise,  making 
an  increased  demand  on  the  heart,  or  through  mental  excite- 
ment, producing  an  increased  quantity  of  pressor  internal  se- 
cretions which  stimulate  the  heart.  The  vaso-motors,  by  con- 
traction, cause  local  rises  of  pressure,  but  normally  they  do  not 
afTect  the  general  systemic  pressure.  Sleep  and  the  recumbent 
posture  lower  systemic  pressure  by  quieting  the  heart. 

Physiologic  elevations  of  blood  pressure  manifest  themselves 
almost  wholly  in  the  systolic  phase ;  that  is  to  say,  they  are 
really  increases  in  the  pulse  pressure.  A  normal  person  may 
take  the  most  vigorous  exercise,  or  undergo  the  most  marked 
mental  excitement,  without  any  perceptible  change  in  his  dias- 
tolic pressure.  Such  elevations,  moreover,  are  transient ;  they 
quickly  subside  as  soon  as  the  cause  is  removed. 

PATHOLOGIC  VARIATIONS. 

Pathological  variations  of  blood  pressure  follow  the  same 
rule  as  physiological,  namely,  elevations  are  caused  by  any 
conditions  which  increase  either  the  force  of  the  heart  or  the 
resistance  of  the  vessels,  low  pressures  by  those  which  dimin- 
ish either  or  both  of  these  factors. 

HYPERTENSION. 

High  blood  pressure  due  to  incrc-ased  heart  force,  even 
when  the  cause  is  pathologic,  usually  manifests  itself  in  the 
systolic  phase  only,  i.  e.  it  is  an  increase  of  pulse  pressure; 
and  tends  to  fall  back  to  normal  when  the  cause  is  removed. 
High  pressure  of  this  type  is  usually  found  in  (1)  acute  tox- 
emias, which  \  iolently  stimulate  the  heart  muscle,  known  as 
asthenic  fevers,  and  (2)  certain  diseases  involving  the  ductless 
glands,  where  there  is  either  an  excess  of  pressor  secretions 
or  a  deficiency  {}f  depressor  secretions  thrown  into  the  blood, 
acting  u])(jn  the  heart  by  way  of  the  sympatlutic  mcin  es. 

High  pressure  due  to  increased  resistance  in  the  vessels 
manifests  itself  primarily  in  the  diastolic  or  static  phase,  later 
and  sec(jndarily  in  the  systolic.  There  is  but  one  explanation 
of  a  marked  and  continuous  elevation  of  static  ])ressure, 
nanicl}',    a    lon^  continued    absorption    of    Ktw-L;rade    t<»\iiu's, 


BLOOD  PRESSURE  77 

which  do  not  stimulate  the  heart,  but  which  gradually  replace 
the  elastic  tissues  of  the  vessels  with  inelastic,  fibrous  tissue 
(arteriosclerosis),  thus  increasing  their  resistance  to  the 
blood-stream.  To  cope  with  this  increased  resistance,  the 
heart  is  obliged  to  develop  a  progressively  increasing  amount 
of  energy,  (which  it  does  by  enlarging  its  muscle),  so  that 
these  cases  eventually  exhibit  very  high  systolic  pressures. 
The  systolic  pressure,  indeed,  rises  in  a  much  greater  ratio 
than  the  diastolic. 

The  important  feature  in  such  cases,  however,  is  the  static 
hypertension.  The  high  systolic  pressure  is  a  compensatory 
process.  A  patient  whose  static  pressure  is  high,  showing 
increased  resistance  in  the  vessels,  must  develop  a  proportion- 
ately high  pulse  pressure  in  order  to  function  adequately.  The 
gravest  cases  of  all  are  those  in  which  the  static  pressure  is 
high  and  the  pulse  pressure  is  not  proportionately  raised,  show- 
ing that  the  heart  is  not  compensating  for  the  increased  resist- 
ance. Such  a  state  of  affairs  is  a  sign  of  a  weakened,  failing 
heart,  and  is  the  precursor  of  disaster. 

HYPOTENSION. 

Low  blood  pressure  is  generally  due  to  certain  low-grade 
chronic  infections  which  weaken,  without  hardening,  the 
walls  of  the  vessels,  and  also  the  heart  muscle.  All  the  asthenic 
fevers  do  this  temporarily — typhoid,  typhus,  diphtheria,  etc. 
Tuberculosis  is  the  chief  permanent  cause  of  low  pressure.  It 
is  also  found  in  certain  diseases  involving  the  ductless  glands, 
in  which  there  is  either  an  excess  of  depressor  or  a  deficiency 
of  pressor  secretions,  as  in  certain  types  of  goitre,  premature 
menopause,  etc.  Both  static  and  dynamic  pressures  are  usu- 
ally low,  and  the  pulse  pressure  as  well. 

BLOOD  PRESSURE  AND  THE  EYE. 

The  eye  bears  a  direct  and  intimate  relation  to  conditions 
of  blood  pressure,  especially  to  hypertension,  and  this  relation 
may  be  said  to  have  two  general  modes  of  expression : 

(1)  The  disturbed  tension,  per  se,  impairs  the  functioning 
of  the  eye, 

(2)  The  structures  of  the  eye  share  in  the  pathologic  pro- 
cesses which  cause,  and  which  result  from,  the  disturbed 
tension. 


78  BLOOD  PRESSURE 

1.  Hypertension,  especially,  impairs  the  functioning  of  the 
eye,  both  as  to  its  muscular  and  its  retinal  elements,  causing 
insufficiency  of  accommodation  and  diminished  visual  acuity. 
Even  the  temporary  high  {pressures  due  to  physiologic  causes 
frequently  produce  transient  disturbances  of  this  kind.  Almost 
everyone  has  experienced  blurred  vision,  and  especially  ina- 
bility to  read  at  near  point,  for  quite  a  few  minutes  after 
engaging  in  violent  exercise,  until  the  heart  quiets  down  and 
blood  i)ressure  falls  again. 

In  ])athologic  hypertension  the  ocular  disturbances  are  less 
turbulent,  because  the  high  pressure,  as  a  rule,  comes  on 
gradually,  so  that  the  patient  often  has  no  subjective  knowl- 
edge of  visual  deficiency,  or  attributes  it  to  error  of  refraction, 
until  a  visit  to  the  refractionist  and  a  test  of  the  blood  pressure 
reveals  the  true  state  of  afifairs.  On  the  other  hand,  the  ocular 
effects  of  pathologic  hypertension  arc  more  permanent  and 
far-reaching  than  those  of  physiologic  elevations;  and,  as  a 
matter  of  fact,  the  eye  is  one  of  the  earliest  organs  to  expe- 
rience the  ill  C()nse(|uences  of  high  blood  i)ressure,  because  the 
high  elastic  coefficient  (incompressibility)  of  the  vitreous  i)er- 
mits  the  high  jjressure  in  the  ocular  arteries  to  be  transmitted 
almost  in  full  to  the  ciliary  body  and  the  retina. 

Insufficiency  of  accommodation  (premature  presbyopia) 
and  low  visual  acuity,  then,  may  be,  and  often  are.  due  to 
sheer  hypertension,  i.  e.  increase  of  pulse  pressure,  where  the 
static  pressure  is  normal,  and  no  destructive  processes  are 
present  or  discernible  either  in  the  eye  itself  or  in  the  systemic 
vessels.  And — what  is  of  great  imj)ortance  to  the  refraction- 
ist— these  visual  disturbances,  due  to  high  pressure,  are  quite 
often  amenable  to  help  by  lenses,  affording  a  som\e  of  ilecep- 
tion  and  misleading  to  both  operator  and  patient.  A  hyperope. 
for  instance,  who,  by  reason  of  hypertension,  is  unable  to  per- 
form ])r(jper  facultative  accommodation,  may  \ery  easily  be 
enabled  U)  read  well  at  JO  feet  by  means  of  plus  correction  ; 
or  a  patient  whose  reading  accommodation  is  deficient  because 
of  hyi)ertension  may  well  be  enabled  to  read  with  ease  by 
plus  lenses;  thus  deluding  the  refractionist  into  the  idea  that 
he  has  remedied  (he  trouble. 


BLOOD  PRESSURE  79 

It  may  be  laid  down  as  a  rule  of  practice,  therefore,  that 
when  insufficiency  of  accommodation  and  poor  visual  acuity 
are  associated  with  high  blood  pressure,  the  latter  condition 
is  to  be  dealt  with  before  the  refractionist's  w^ork  can  be  ade- 
quately performed. 

Hypotension,  per  se,  does  not,  as  a  rule,  produce  any  dis- 
tinctive ocular  symptoms.  The  eye  shares  with  the  rest  of  the 
organism  in  the  poor  nutrition  due  to  low  pressure,  so  that  it 
is  quite  possible  to  have  accommodative  insufficiency  from 
hypotension  as  well  as  from  hypertension.  The  retina  is  quan- 
titively  anemic,  and  therefore  not  so  keen  in  its  reactions  as 
normally.  And  the  extrinsic  muscles,  being  weak,  show  low- 
ered duction  powers.  But  in  many  patients  with  low  blood 
pressure  no  abnormal  functioning  of  the  eye  is  discoverable. 

2.  ^^'hen  hypertension  is  the  outcome  of  increased  resist- 
ance in  the  blood  vessels,  manifesting  itself  primarily  in  the 
static  phase,  then,  sooner  or  later,  the  vessels  of  the  eye  share 
in  the  pathologic  process  which  underlies  the  whole  disturb- 
ance, and  in  addition  to  the  disturbances  due  to  the  high  ten- 
sion, we  have  a  train  of  organic  disease  in  the  eye  which  still 
further,  and  more  seriously,  interferes  with  vision, — arterio- 
sclerosis, retinitis,  retinal  hemorrhages,  retinal  thrombosis, 
choroiditis,  etc.  No  doubt  many  cases  of  glaucoma  have  their 
rise  in  such  conditions. 

All  of  the  chronic  infectious  diseases  which  harden  the 
blood  vessels  and  raise  the  diastolic  pressure  sooner  or  later 
register  themselves  in  the  eye.  Nephritis,  syphilis,  gall-blad- 
der infection,  gastro-intestinal  toxosis,  pyorrhea,  all  these  and 
many  others  are  in  this  class.  The  eye,  being  an  end  organ, 
(i.  e.  a  terminal  loop  in  the  circulatory  system),  is  bound  to 
sufifer  from  toxemia ;  often  it  is  the  earliest  organ  to  give  sub- 
jective notice  of  the  trouble;  and  quite  frequently  such  condi- 
tions are  discovered  in  the  eye-ground  before  there  are  any 
subjective  symptoms  at  all.  In  all  such  cases,  the  finding  of  a 
high  blood  pressure  (static)  confirms  the  evidence  of  the 
ophthalmoscope. 

Hypotension  is  much  less  often  associated  with  organic  dis- 
ease of  the  eye  than  hypertension.  Most  low  pressures  are 
due  to  disturbances  of  the  internal  secretions,  and  these  do 


80  BODAL'S  TEST 

not  become  organic.  Occasionally  a  continued  fever  of  the 
asthenic  type,  like  typhoid,  produces  retinal  and  ciliary 
troubles.  Anemia,  from  whatever  cause,  is  attended  by  retinal 
anemia,  sometimes  even  to  the  point  of  tiny  hemorrhages.  In 
general,  however,  it  may  be  said  that  long-continued  low 
pressure,  associated  with  organic  changes  in  the  ocular  struc- 
tures, indicates  tuberculosis. 

Bodal's  Test.    A  test  for  color  vision  by  means  of  colored  blocks. 

Bowman's  Membrane.  The  second  layer  of  the  cornea,  imme- 
diately under  the  external  epithelium.  It  is  also  known  as  the 
external  limiting  membrane. 

Bozzi's  Foramen.     Macula  lutea  of  tlie  retina. 

Brachymetropia.  This  term  (literally,  a  shortness  of  the  eye) 
denotes  the  physical  condition  of  the  eye  which  constitutes 
hyperopia. 

Bulbus  Oculi.     The  eyeball. 

Bumke's  Pupil.  Dilatation  of  the  pupil  in  response  to  psychic 
stimuli.     It  is  absent  in  dementia  precox. 

Buphthalmia.     Same  as  buphlhalmos. 

Bupthalmos.  Congenital  glaucoma,  lixdrophthalnins,  keratoglo- 
bus ;  increase  of  intracjcular  fluid,  i)roducing  enlargement  of 
the  eye. 

Burns'  Amaurosis.  Amaurosis  due  to  sexual  excesses.  See 
Amaurosis. 

Butler's  Shield.  A  watchglass  so  fastened  to  adhesive  plaster 
as  to  protect  the  unalTected  eye  in  case  of  purulent  ophthalmia. 

Caecitas.     J'lindness. 

Camera.    Another  name  for  the  chambers  of  the  eye. 

Campimeter.  An  in^trunuiit  for  drtii  niinini^  the  ticld  of  vision. 
See  Perimeter. 


CANALS  81 

Canals.  These  are  tiny  channels  in  the  eye  which  lead  from  one 
open  space  into  another.     They  are  named  as  follows: 

Central  Canal.  A  slightly  compressed  tubular  channel  1  to 
2  mm.  in  diameter,  from  the  retinal  papilla  to  the  posterior 
pole  of  the  lens.  This  canal,  in  the  adult,  is  usually  a  blind 
duct,  being  the  remains  of  the  fetal  channel  which  conducted 
the  hyaloid  artery.  Occasionally  the  artery  itself  persists  in 
the  adult. 

Hyaloid  Canal.     Same  as  the  central  canal. 

Lacrymal  Canal,  or  Ducts.  A  channel  running  from  the 
lower  palpebral  sac,  a  few  mm.  from  the  inner  canthus,  to  the 
inferior  meatus  of  the  nose.  The  canal  is  really  a  combination 
of  two  ducts,  which  are  continuous  with  each  other, — the 
lacrymal  duct,  in  soft  tissue,  from  the  sac  to  the  superior 
maxillary  bone,  and  the  nasal  duct,  through  the  maxillary 
bone,  from  there  to  the  meatus.  Its  purpose  is  to  drain  the 
tears  from  the  eye. 

Canal  of  Cloquet.     Same  as  the  central  canal. 

Canal  of  Petit.  A  supposed  canal  extending  about  the 
crystalline  lens  in  the  folds  of  the  suspensory  ligament. 

Canal  of  Schlemm.  A  circular  canal,  in  the  deep  scleral 
tissue,  surrounding  the  cornea.  It  is  supposed  to  be  a  part  of 
the  lymphatic  system,  although  there  is  a  great  deal  of  con- 
troversy concerning  it. 

Canal  of  Stilling.     Same  as  the  central  canal. 

Canthoplasty.  Any  operation  which  has  for  its  object  changing 
the  condition  of  the  eyelid. 

Canthororrhaphy.     An  operation  for  sewing  the  eyelids. 

Canthotomy.    Slitting  of  the  canthus. 

Canthus.  The  angle  made  by  the  junction  of  the  upper  and 
lower  eyelids.  The  one  on  the  nasal  side  is  called  the  inner, 
and  the  one  on  the  temporal  side  the  outer,  canthus. 

Capsule.     A  sac,  or  membrane,   which  surrounds  and  contains 
an  organ  or  tissue.    The  capsules  of  the  eye  are  as  follows: 
Bonnet's  Capsule.    See  Capsule  of  Tenon. 


82  CAPSULOTOMY 

Capsule  of  the  l.ens.  The  sac  which  surrounds  and  con- 
tains the  crystalline  lens.  Its  index  of  refraction  is  1.39.  It  is 
made  of  elastic  fibres  which  permit  of  a  chan,i2:c  of  curvature 
when  the  ciliary  muscle  contracts  (see  Accommodation)  but 
which  gradually  lose  their  elasticity  as  age  advances  until 
this  change  is  no  longer  possible.  In  operations  for  cataract 
the  capsule  is  usually  opened  and  the  lens  extracted  from  it ; 
the  capsule  then  absorbs. 

Capsule  of  Tenon.  A  delicate  membrane  which  surrounds 
the  eyeball  from  the  optic  nerve  posteriorly  to  within  a  few 
millimeters  of  the  corneal  ring  anteriorly. 

Capsulotomy.  Incision  through  the  capsule  of  the  lens  in  an 
oj)eration. 

Cardinal  Points.    See  Points. 

Cartilage,  Tarsal.  The  stiff  tissue  between  the  skin  and  the 
muscle  of  the  eyelid  which  gives  it  shape  and  support. 

Caruncula.  A  small  reddish  body  near  the  inner  canthus  of  the 
eye. 

Cast.     .\  lay  term  for  strabismus. 

Cataphoria.  A  state  of  muscular  imbalance  in  which  the  eye 
tends  to  turn  downward,  duo  tcj  the  dominance  of  the  inferior 
rectus  muscle.     (See  Heterophoria.) 

Cataract.  The  crystalline  lens,  being  a  part  of  the  retracting 
media  of  the  eye,  must  necessarily  be  transparent  in  order  to 
function  properly.  Under  certain  pathologic  conditions,  which 
are  not  always  well  understood,  the  lens  loses  its  transparencx', 
and  becomes  yellow  and  opa(|ue,  causing,  of  course,  a  degree  of 
Vilindness  proportionate  to  its  opacity.  This  cnndition  is 
known  as  cataract. 

CAUSES  OF  CATARACT.. 
As  intimated,  it  is  not  always  easy,  or  even  possible,  {o 
determine  the  underlying  cause  in  a  given  case  of  cataract. 
Its  primary  ca<use,  to  be  sure,  is  interference  with  the  nutri- 
ment of  the  lens;  and  the  peculiar  anatomy  and  physiology  of 
the  lens,  i.  c.  its  lack  of  blood  siipplx   .md  the  ileiiNation  of  its 


CATARACT  83 

nourishment  from  the  surrounding  ciliary  body,  renders  it 
very  susceptible  to  such  interference. 

It  has  recently  been  determined,  beyond  question,  that  one 
of  the  principal  causes  of  the  origin  of  cataract  is  the  influence 
of  light  which  is  rich  in  ultra-violet  rays,  such  as  daylight, 
which  interferes  with  the  nutrition  of  the  lens,  not  by  direct 
action,  but  through  the  optical  heterogeneity  of  the  lens, 
which  disperses  a  part  of  the  ultra-violet  rays  to  the  ciliary 
processes,  damaging  their  epithelium.  That  this  can  only  be 
a  primary,  and  not  an  underlying  cause,  however,  is  evident 
from  the  fact  that  everyone  who  is  exposed  to  white  light  does 
not  by  any  means  acquire  cataract. 

As  contributing  causes  may  be  assigned,  in  general,  all  con- 
ditions which  impair  the  nutrition  of  the  body  as  a  whole, 
among  which  must  be  included  old  age,  and  those  which  im- 
pair the  local  nutrition  of  the  lens  itself.  In  the  first  class  of 
cases,  in  addition  to  senility,  we  have  certain  systemic  dis- 
eases, notably  diabetes,  nephritis,  heart  disease,  arteriosclero- 
sis, autotoxemia,  malaria ;  in  the  second  class,  blows  upon  the 
eye  (either  as  an  immediate  or  a  remote  effect),  violent  in- 
flammations of  the  anterior  portion  of  the  eye,  myopia  of  high 
degree,  retinitis  pigmentosa,  chorioditis,  glaucoma. 

VARIETIES  OF  CATARACT. 

Cataracts  may  be  classified  according  to  (1)  their  pathology, 
or  (2)  their  position  and  structure. 

(1)  According  to  their  etiology  and  pathology,  the  principal 
forms  of  cataract  are : 

(a)  Congenital.  Caused  by  disturbances  of  development  or 
by  an  inflammation  of  the  eye  in  utero.  Usually  bilateral  (in 
both  eyes),  and  frequently  inherited.  They  are  not  usually 
made  out  until  the  child  is  some  weeks  old. 

(b)  Senile.  By  far  the  commonest  form.  Rarely  occur 
1)efore  fifty  years  of  age,  but  occasionally  between  forty  and 
fifty.  Usually  affect  both  eyes,  but  seldom  at  the  same  time, 
one  eye  being  usually  ahead  of  the  other  in  its  development. 
The  mere  fact  of  a  cataract  occuring  in  an  old  person,  of  course, 
is  not  sufficient  to  diagnose  a  senile  cataract,  as  an  old  person 
may  have  cataract  due  to  other  causes. 


84  CATARACT 

(c)  Diabetic.  Always  bilateral,  and  often  soft  in  character, 
i.e.  show  no  disposition  to  harden  or  ripen. 

(d)  Glaucomatous.  Due  to  the  increased  tension  of  glau- 
coma.    Generally  unilateral,  as  glaucoma  is. 

(e)  Complicated.  This  form  is  due  to,  and  complicated 
with,  inflammations  of  the  eye  or  degenerative  processes. 
Glaucomatous  cataracts  really  belong  in  this  variety,  but  for 
clinical  reasons  are  considered  separately. 

(f)  Traumatic.  In  this  class  we  do  not  include  those  cata- 
racts which  occur  as  the  remote  eflfects  of  injury,  but  only 
those  which  result  directly  and  immediately  from  rupture  of 
the  lens  capsule,  by  a  blow  or  other  injury.  The  aqueous 
humor  enters  the  capsule,  attacks  the  lens  substance,  first 
causes  it  to  swell  and  become  cloudy  and  opaque,  and  finally 
absorbs  it. 

Classified  according  to  their  position  or  area,  the  most  com- 
monly met  forms  are: 

(a)  Anterior  Polar,  situated  in  the  anterior  pole  of  the  lens. 

(b)  Posterior  Polar,  located  in  the  posterior  pole. 

(c)  Nuclear,  situated  in  the  nucleus  of  the  lens. 

(d)  Perinuclear,  or  Lamellar,  due  to  cataract  lying  between 
the  nucleus  and  the  cortex.  Usually  stellar  in  shape.  The 
most  frequent  form  in  children. 

(e)  Circumscribed,  in  which  there  are  limited  opacities  of 
various  shapes  within  the  lens,  chief  of  which  are  central,  where 
there  is  a  small  spherical  opacity  in  the  centre  of  the  lens ; 
fusiform,  where  the  opaciue  lines  run  in  spindle  fashion  in  the 
direction  of  the  axis  of  the  lens;  and  punctate,  where  there  are 
several  spots  distributed  through  the  lens. 

The  above  classification  of  area  applies,  as  a  rule,  only  to 
the  starting-place  of  the  cataract,  as  all  cataracts  tend  to 
spread  until  they  occupy  the  entire  lens. 

STAGES  OF  CATARACT. 

Various  well-defined  stages  in  the  dev  el(>i»ment  of  cataract 
are  recognized  as   follows: 

(a)  Incipient.  Where  the  opacity  has  not  >  ct  reached  the 
stage  of  definite  outline.  In  this  stage  the  cataract  can 
usually  be  diagnosed  only  by  indirect  nicaiis,  i-spcciaily  by  the 


CATARACT  85 

inequality  of  refractiveness  of  different  portions  of  the  lens, 
as  shown  by  the  ophthalmoscope. 

(b)  Intumescent.  The  stage,  after  the  definite  establish- 
ment of  opacity,  during  which  the  cataract  is  ripening,  i.  e. 
until  it  reaches  the  anterior  capsule.  During  this  stage  a  light 
held  near  the  eye,  to  one  side  of  it,  throws  a  shadow  of  the 
iris. 

(c)  Mature.  When  the  anterior  chamber  again  becomes  of 
normal  depth  and  the  iris  no  longer  casts  a  shadow.  The 
cataract  now  has  become  total,  and  is  ready  for  extraction. 

(b)  Hypermature.  If  a  cataract  is  allowed  to  remain  after 
becoming  ripe,  one  of  two  things  happens  to  it:  (1)  It  loses 
water  and  becomes  shrunken  and  disintegrated,  and  the  an- 
terior chamber  becomes  deeper  and  deeper,  or  (2)  It  grows 
more  fluid  in  proportion  as  it  breaks  up  into  smaller  parts,  so 
that  at  last  there  is  a  central  mass  of  solid  surrounded  by 
fluid  (Morgagnian  cataract). 

SYMPTOMS. 

The  subjective  symptoms  are,  of  course,  disturbances  of 
vision,  varying  with  the  nature  and  stage  of  the  disease.  Often, 
in  the  incipient  stage,  there  is  multiple  vision,  due  to  the  opti- 
cal irregularities.  Later,  there  are  scotomata  corresponding  to 
the  areas  of  opacity,  or  general  diminution  of  vision,  if  the 
cataract  is  disseminated.  Eventually,  there  is  complete  blind- 
ness to  all  but  light. 

Objectively,  the  earliest  sign  of  cataract  is  often  the  demon- 
stration of  myopia  in  a  formerly  emmetropic  or  hyperopic 
eye,  due  to  swelling  of  the  lens  ("second  sight"),  or  differences 
of  refractivity  in  different  parts  of  the  crystalline  lens,  shown 
by  the  ophthalmoscope.  Later,  the  opacities  can  be  demon- 
strated, showing  gray  under  oblique  illumination,  black  with 
the  ophthalmoscope.  Anterior  and  posterior  polar  cataracts 
are  differentiated  by  the  parallax  motion ;  on  rotating  the 
ophthalmoscope,  anterior  opacities  move  with,  posterior 
against,  the  mirror. 

When  the  cataract  is  ripe,  the  patient  should  still  be  able  to 
perceive  a  moderately  bright  light  (perception)  and  to  indi- 
cate the  direction  from  which  the  light  is  coming  (projection). 


86  CATARACT 

If  these  two   faculties  are  present,  one  may  hope   for  good 
vision  after  the  cataract  is  extracted. 

TREATMENT. 

If  taken  in  its  incipiency,  the  progress  of  a  cataract  can  un- 
doubtedly be  checked,  in  some  instances,  by  the  wearing  of 
amber  lenses,  to  protect  the  eye  from  white  light,  and  the 
instillation  of  dionin,  to  promote  absorption  and  nutrition. 
Seldom,  however,  do  cataracts  come  under  observation  in 
time  to  carry  out  this  abortive  treatment ;  and  practically  all 
cataracts  progress,  in  spite  of  all  treatment,  to  completion, 
where  extraction  is  the  only  available  remedy.  The  time  re- 
quired for  their  ripening  varies  considerably,  all  the  way  from 
a  few  months  to  two  or  three  years. 

In  the  meantime,  great  assistance  and  comfort  can  be  given 
the  patient  by  a  careful  refraction  of  the  eyes,  and  the  pre- 
scribing of  suitable  glasses,  changing  them  from  time  to  time, 
as  the  changes  in  the  eyes  re(|uire,  until  blindness  becomes  too 
pronounced  for  any  lenses  to  help.  As  stated  above,  the  gen- 
eral tendency  of  cataract  is  to  make  the  eye  more  myopic  than 
it  was  before.  At  the  same  time  it  impairs  the  elasticity  of 
the  accommodative  mechanism,  so  that  the  patient  usually 
needs  more  than  ordinary  presbyopic  correction.  By  skillful 
attention  to  the  patient's  refractive  needs,  useful  vision  can  be 
prolonged  far  into  the  course  of  the  disease. 

The  task  of  extraction  belongs,  of  course,  to  the  oculist, 
and  is  far  too  long  and  complicated  a  subject  to  go  into  here. 
1  he  reader  is  referred  to  a  work  on  ocular  surgery. 

After  extraction  the  real  work  of  the  refractionist  begins. 
It  then  becomes  necessary  to  furnish  the  patient  with  a  plus 
lens  which  will  compensate,  optically,  for  the  loss  of  the  cry- 
stalline;  and  to  this  must,  in  practically  every  case,  be  added 
cylindrical  correction  to  remedy  the  astigmatism  due  to  the 
cicatrisation  of  the  wound  in  the  cornea.  Since  the  adoption 
of  the  scleral  incision,  this  astigmatism  is  not  si^  fre(|uenl  or 
so  marked  ;  but  many  operators  still  use  the  corneal  incision. 
It  is  best  not  to  attempt  final  refraction  of  the  eye  until  several 
months  after  operation,  so  as  to  permit  it  to  settle  down  to 
its  permanent  state. 


CATARACT-SPOON  87 

Cataract-Spoon.  An  instrument  shaped  like  a  spoon,  used  for 
removing  the  lens. 

Catatropia.  An  unusual  muscular  anomaly  in  which  a  hypo- 
tropia  of  one  eye  alternates  with  that  of  the  other. 

Catoptrics.  That  branch  of  optics  which  deals  with  incident  and 
reflected  light,  based  upon  the  law  that  the  angle  of  incidence 
and  the  angle  of  reflection  are  equal. 

Catoptric  Test.  A  test  to  determine  the  presence  of  a  cataract 
by  reflection  of  light.  If  a  candle  be  held  about  a  foot  from 
the  eye,  about  30  deg.  from^the  optical  centre,  and  the  observer 
stand  the  same  distance  on  the  opposite  side,  if  no  cataract  be 
present,  three  images  of  the  candle-flame  will  be  seen,  one 
reflected  by  the  surface  of  the  cornea,  one  by  the  anterior  sur- 
face of  the  lens,  and  one  by  the  posterior  surface  of  the  lens ; 
if  there  be  cataract,  the  latter  image  will  of  course  be  missing. 

Cat's  Eye  Pupil.  A  pupil  the  aperture  of  which  is  horizontally 
long  and  narrow. 

Caustic.  In  optics  this  name  is  given  to  a  curve  to  which  the 
rays  of  light  reflected  or  refracted  by  another  curve  are  tan- 
gent. Caustics  are  therefore  of  two  kinds,  catacaustics,  being 
caustics  by  reflection,  and  diacaustics,  being  caustics  by  re- 
fraction. 

Cavascope.     Device  for  lightng  the  interior  of  cavities. 

Cecita.     Blindness,  same  as  cecity. 

Cedmatophthalmia.     Ophthalmia  caused  by  rheumatism. 

Celoscope.     Instrument  for  illuminating  cavities  of  the  body. 

Centimeter.  One-hundredth  part  of  a  meter.  Ten  millimeters. 
Equivalent  to  about  two-fifths  of  an  inch. 

Centrad.  The  hundredth  part  of  a  radian,  used  as  a  unit  of 
measurement  in  prismometry.     See  Prism. 

Centrage.  Designating  a  condition  in  which  the  center  of  all 
the  refracting  surfaces  of  the  eye  take  a  position  in  one 
straight  line. 


88  CENTROPHOSE 

Centrophose.  Sensation  of  a  central  dark  spot  along  the  line  of 
vision. 

Centre.  There  are  but  two  geometric  figures  which  can  prop- 
erly be  said  to  have  a  centre — the  circle  in  plane  geometry, 
and  the  sphere  in  solid  geometry.  Centres,  in  optics,  there- 
fore, are  points  in  the  centre  of  spherical  bodies  or  systems,  or, 
in  the  case  of  segments,  at  the  centre  of  the  spheres  of  which 
they  are  segments. 

Centre  of  Curvature.  The  center  of  the  sphere  of  which  a 
lens  curvature  is  the  segment. 

Optical  Centre.  The  point  where  the  secondary  axes  of  a 
refracting  system  meet  and  cross  the  principal  a.xis.  This 
corresponds  to  the  geometric  centre  of  the  refracting  system 
considered  as  a  single  sphere.  In  thin  lenses,  and  for  prac- 
tical purposes  of  calculation,  the  nodal  points  are  virtually 
coincident  with  and  constitute  the  optical  centre.  (See  Point, 
Nodal). 

Centre  of  Rotation.  The  point  around  which  the  eyeball  is 
supposed  to  rotate  under  the  action  of  the  extrinsic  muscles. 
The  location  of  this  point  is  a  matter  of  uncertainty. 

The  w^ord  Centre  is  also  used  as  a  verl)  in  optics,  signifying 
to  place  the  lens  before  the  eye,  or  in  its  mounting,  in  such  a 
way  that  the  visual  axis  (q.  v.)  will  pierce  the  optical  centre 
of  the  lens.  This  is  routine.  More  interest  attaches  to  those 
cases  in  which  it  is  desired  to  place  the  lenses  before  the  eye 
so  as  not  to  be  centered.     (See  DecenteringV 

Again,  the  word  Centre  is  em])loye(l  in  connection  with  the 
nervous  system  to  describe  the  point  in  the  brain  or  spinal 
cord  from  which  a  nerve  arises  or  in  which  it  terminates. 

Centrifugal.  A  term  applied  to  a  set  of  radiating  forces  which 
act  from  the  circumference  toward  a  common  center  out 
toward  the  circumference.  Centrifugal  light  waves  are  plus 
waves. 

Centripetal.  .Applied  to  a  set  of  radiating  forces  which  act  from 
the  circumference  toward  a  common  center.  Centripetal  light 
waves  are  minus  waves. 


CERATOCELE  89 

Ceratocele.     Bulging  of  the  cornea  due  to  a  tumor. 

Cerebroscope.  Cerebroscopy.  Applied  to  the  use  of  the  oph- 
thalmoscope for  the  purpose  of  diagnosing  conditions  of  the 
brain. 

Chalazion.  Infection  and  enlargement  of  a  meibomian  gland, 
(q.  v.).  Also  called  a  meibomian  cyst,  and  a  tarsal  cyst. 
Usually  appears  on  the  margin  of  the  upper  lid.  Diagnosis  and 
treatment  belong  to  medicine.  They  seem  to  be  at  least  ag- 
gravated by  errors  of  refraction. 

Chambers.  The  spaces  in  the  eye  between  the  cornea  and  the 
anterior  surface  of  the  lens,  containing  the  aqueous  humor. 
The  anterior  chamber  extends  from  the  cornea  to  the  iris,  the 
posterior  chamber  from  the  iris  to  the  lens.  Many  anatomists 
deny  the  existence  of  the  posterior  chamber,  stating  that  there 
is  but  one  chamber,  extending  from  the  cornea  to  the  lens. 

Check  Ligaments.    See  Ligaments. 

Chemosis.  Swelling,  or  edema,  of  the  eyelid  by  the  extravasa- 
tion of  blood  serum  into  the  loose  tissue.  It  is  a  symptom  of 
some  other  underlying  trouble,  for  the  medical  man  to  discover. 

Chiasm.  The  place  where  the  partial  decussation  of  the  optic 
tract  takes  place.  As  the  fibres  of  the  optic  nerves  from  the 
two  eyes  proceed  backward  to  the  brain,  those  which  originate 
at  the  inner  half  of  each  retina  cross  over  to  the  opposite  side, 
while  those  which  come  from  the  outer  half  of  each  retina 
proceed  on  the  same  side.  Thus,  after  passing  the  optic  chiasm, 
the  entire  lateral  half  of  each  retina  is  represented  on  the  cor- 
responding side  of  the  brain.  There  are  a  few  nerve  fibres 
which,  at  the  chiasm,  cross  over  and  go  back  to  the  eye  on  the 
opposite  side.  These  are  called  consensual  fibres  and  are  re- 
sponsible for  the  consensual  reflex.     (See  Reflexes.) 

The  optic  chiasm  is  a  most  interesting  and  important  point 
in  the  optic  tract.  Injury  to  the  chiasm,  or  tumor  of  the  chiasm, 
usually  gives  half-blindness  on  the  nasal  half  of  each  retina, 
because  such  injury  or  growth  is  usually  in  the  middle  of  the 
chiasm,  affecting  the  inner  optic  nerve  fibres  as  they  cross. 
(See  Optic  Tract.) 


90  CHIASMAL  IMAGE 

Chiasmal  Image.  According  to  Prentice,  "a  strictly  figurative 
image  consisting  of  that  orderly  assemblage  of  the  optic-nerve 
fibrils,  within  the  cross-sectional  and  comparatively  small  area 
of  the  optic  chiasm,  which  receive  their  individual  stimuli  from 
corresponding  points  in  each  retinal  image."  He  deduces  the 
rule  that  for  1  prism  dioptre  of  deviation  of  the  visual  axes 
tiiere  is  a  separation  of  0.15  mm.  between  the  chiasmal  image- 
centers. 

Chiastometer.  An  instrument  devised  by  Landolt  for  determin- 
ing the  distance  between  the  two  eyes  during  convergence. 

Chionablepsia.    Snow-blindness. 

Chloropia.     Vision  in  which  objects  appear  green. 

Chloropsia.     Same  as  chloropia. 

Choked  Disc.    See  Disc. 

Chorea.    .See  St.  Vitus'  Dance. 

Choriocapillaris.     1'lie  middle  layer  of  the  choroid  coat. 

Chorioretinal.  Pertaining  to  the  retina  and  the  choroid  coating 
of  the  eye. 

Choroid.  Chorioid.  A  part  of  tlie  second,  or  vascular  tunic  of 
the  eyeball,  lining  the  sclera  from  the  optic  ner\e  heatl  to  the 
ciliary  body,  with  which  it  is  continuous.  It  is  closely  attached 
to  the  sclera  at  the  optic  nerve,  at  the  posterior  pole  (where 
the  posterior  ciliary  arteries  enter)  and  at  the  etjuator  (where 
the  venae  vorticosae  leave  the  globe).  l.\iiii;  ininudiately  be- 
hind the  transparent  retina,  it  forms  an  important  part  of  the 
fundus  picture  seen  with  the  oi)hthalnu)Sct)pe. 
The  choroid  consists  of  ti\e  laNcrs,  as  follows: 

1.  The  .Su|)rac!ioroi(l.  a  non-vascular,  pij^incntcil  membrane 
between  tlie  choroid  and  the  sclera. 

2.  Layer  of    Larger   Vessels,  chielly   \eins.  the  interspaces 
being  abundantly  filled  with  brown  pigment. 

3.  Layer  of  Medium-si/ed  \essels.    This  layer  is  extremely 
thin  and  only  slightly  |)igmented. 


CHOROID  91 

4.  Layer  of  Capillaries.     This  layer  has  no  pigment. 

5.  Lamina  Vitrea,  or  Basalis,  a  homogeneous  membranous 
coating  over  the  choroid. 

The  blood  supply  of  the  choroid  is  obtained  principally  by 
way  of  the  short  posterior  ciliary  arteries.  A  few  branches 
from  the  long  posterior  ciliaries  reach  it.  The  blood  flows  off 
from  the  choroid  by  numerous  veins,  which  keep  coalescing  to 
form  large  trunks,  eventually  combining  into  four  or  five  vor- 
tices behind  the  equator,  and  these,  in  turn,  empty  into  the  two 
\  enae  vorticosae,  which  penetrate  the  sclera  and  leave  the  eye- 
ball. 

The  pigment  with  which  the  uveal  structures  are  so  abund- 
antly supplied  is  well  represented  in  the  choroid.  Here  the 
pigment  is  of  a  yellow-brown  color,  which  helps  to  give  the 
characteristic  hue  to  the  fundus. 

Choroido-Cyclitis.     Inflammation  of  choroid  and  uveal  tract. 

Choroido-Iritis.     Inflammation  of  choroid  and  iris. 

Choroido-Retinitis.    Inflammation  of  choroid  and  retina. 

Chromasia.  Color  effect  produced  by  chromatic  aberration  in 
functioning  of  lenses. 

Chromatelopsia.    A  pronounced  degree  of  color  blindness. 

Chromatherapy.  Treatment  by  means  of  lights  of  different 
colors.  ' 

Chromatic  Aberration.     See  Aberration. 

Chromatic  Audition.  Literally,  color-hearing.  A  phenomenon 
in  which,  along  with  the  sensation  of  hearing,  there  emerges 
also  a  sensation  of  color.  For  a  full  discussion  of  this  interest- 
ing phenomenon  the  reader  is  referred  to  a  work  on  psychology. 

Chromatic  Dispersion.    The  splitting  of  white  light  into  its  com- 
posite color  waves  by  the  action  of  a  prism,  as  in  the  spectrum. 
— Anomalous.     That  in  which  the  colors  are  spread  out  in 
an  order  differing  from  that  of  the  regular  spectrum. 


92  CHROMATIC  TEST 

— Irrational.  That  in  which  the  dispersion  of  the  extreme 
colors  of  the  spectrum  is  not  constant  for  different  media. 

— Partial.  As  relating  to  any  pair  of  fixed  lines  in  the  spec- 
trum, for  a  given  substance. 

— Mean.   As  relating  to  two  definite  lines  of  the  spectrum. 

Chromatic  Test.  This  refraction  test  is  based  upon  the  chromatic 
aberration  of  the  eye,  which  is  exaggerated  when  the  focal 
point  of  the  eye  is  either  behind  or  in  front  of  the  retinal  plane, 
i.  e.,  when  the  eye  is  either  hypcropic  or  myopic.  The  aberra- 
tion is  accentuated  and  made  subjectively  perceptible  by  means 
of  a  cobalt  lens,  which  cuts  out  all  the  color  waves  except  the 
two  extreme  colors,  red  and  violet,  i.  e.,  the  waves  of  least  and 
greatest  refrangibility.  The  gradual  merging  of  the  inter- 
mediate colors  being  thus  destroyed,  a  sharp  distinction  is 
perceived  between  the  focussing  of  these  two  colors. 

The  following  rules  have  been  given : 

With  the  cobalt  lens  before  the  eye,  the  patient's  attention 
is  directed  to  a  small  circular  light,  usually  at  infinity. 

In  hyperopia,  the  violet  waves  focus  on  the  retina,  while 
the  red  waves  are  still  unfocusscd ;  hence  the  patient  sees  a 
violet  center  with  a  ring  of  red  around  it ;  and  plus  correction 
is  added  until  he  sees  no  more  red  around  the  center,  or  until 
there  is  a  slight  blue  ring. 

In  myopia,  the  red  waves  focus  on  the  retina,  while  the  blue 
waves  have  focussed  and  are  diverging  again  l)y  the  time  they 
reach  the  retina;  hence  the  patient  sees  a  red  center  with  a 
blue  ring  around  it.  Minus  correction  is  added  until  the  blue 
ring  disappears,  or  is  very  thin. 

In  hyperopic  astigmatism,  the  ]);iti(.iU  sees  red  on  each  side, 
or  in  some  other  nu-ridian.  Add  i)lus  correction  until  llu'  red 
on  each  side  almost  disappears,  then  minus  cylinders,  axis 
where  the  red  shows,  until  the  red  a])pears  at  rij^ht  angles  to 
the  axis  of  the  cylinder,  i.  e.,  until  a  nd  ring  apjji'ars  around 
the  center.  Now  add  plus  splu'res  until  llurc  is  no  longer  a 
red  ring,  but  a  narrow  blue  ring. 

In  myopic  astigmatism,  the  p.itient  si'cs  blue  on  c.ich  side 
correspondinj;  to  the  two  chief  meridians,  and  minus  cyliiuU-rs 
are  to  be  j>ut  on  imtil  there  is  a  red  ring  around  the  center  as 


CHROMATISM  93 

above,  after  which  plus  spheres  are  to  be  added  until  there  is 
no  red  ring,  but  a  narrow  blue  one. 

In  mixed  astigmatism,  the  patient  sees  blue  up  and  down 
and  red  on  each  side,  or  vice  versa.  Put  on  plus  spheres  until 
there  is  a  little  red  left  in  its  place,  then  minus  cylinders,  axis 
where  the  red  is,  until  there  is  a  red  ring.  Thereafter  proceed 
as  above. 

Chromatism.  The  quality  of  exhibiting  chromatic  aberration. 
(See  Aberration.) 

Chromatometer.  An  instrument  for  measuring  perception  of  dif- 
ferent colors.  The  principle  is  to  compare  two  complementary 
colors,  produced  by  a  quartz  plate  cut  at  right  angles  to  its 
optic  axis  between  a  Nicol's  prism  and  an  Iceland  spar.  The 
patient,  if  color  blind,  will  find,  on  turning  the  prism  that  in  a 
certain  position  the  two  complementary  colors  are  equal,  show- 
ing at  once  what  colors  are  confounded  by  him.  Rose  and 
Helmholz  both  invented  instruments  of  this  kind,  which 
Chibret  improved  by  enabling  them  to  give  any  desired  degree 
of  color  saturation.     (See  Color  Blindness.) 

Chromatophobia.     Fear  of  colors. 

Chromatopsia.  An  abnormal  condition  in  which  the  patient  sees 
everything  of  a  certain  color.  The  various  kinds  of  chroma- 
topsia are  named  according  to  the  color  seen,  as  Chloropsia, 
Rhodopsia,  etc.,  q.  v. 

Chromatoptometer.  An  instrument  for  measuring  the  intensity 
of  color. 

Chromophan,.    Retinal  pigment. 

Chromophose.  A  subjective  sensation  of  a  spot  of  color  in  the 
line  of  vision. 

Chromotalopsia.     Color-blindness. 

Chromoptometer.    Same  as  Chromatometer. 

Ciclite.    Same  as  Cyclitis. 

Cilia.    Hairs.    In  regard  to  the  eyes,  the  eye-lashes. 


94  CILIARY 

Ciliary.  Pertaining  to  the  lashes.  The  word  is  also  used  in  a 
descriptive  sense  to  signify  likeness  to  hairs,  as  the  ciliary 
body,  the  ciliary  muscle,  etc. 

Ciliary  Body.  The  middle  part  of  the  second  tunic  of  the  eye, 
so  called  because  of  the  hair-like  appearance  of  its  striata.  It 
has  its  beginning  in  a  flat  portion  (the  orbiculus  ciliaris)  which 
is  continuous  with,  and  almost  indistinguishable  from,  the 
choroid,  except  for  a  different  arrangement  of  the  vessels  and 
the  absence  of  the  ciliary  capillaries,  which  end  at  the  era 
serrata. 

The  largest  portion  of  the  ciliary  body  is  the  ciliary  muscle. 
consisting  of  two  portions,  distinguished  by  the  different  direc- 
tion of  their  fibres:  (1)  The  external,  known  as  Bruck's  muscle, 
containing  longitudinal  or  meridional  fibres,  running  from  the 
corneal  boundary  to  the  choroid,  and  (2)  The  internal,  known 
as  Mueller's  muscle,  containing  circular  fibres,  which  is  the 
active  factor  in  accommodation. 

The  ciliary  processes  are  on  the  muscle,  and  consist  of 
stroma  containing  pigment  substance  and  an  extraordinary 
number  of  blood  vessels.  IMie  ciliary  btxly  is.  in  fact,  the  most 
vascular  part  of  the  uvea. 

The  inner  surface  of  tiie  ciliary  bod)   has  three  layers: 

(1)  The  vitreous  lamina. 

(2)  The  layer  of  pigmented  cells. 

(3)  The  non-pigmented  layer  of  cylindrical  cells.  The  last 
two  layers  (pars  ciliaris  retinae)  pass  o\  er  to  the  posterior 
surface  of  the  iris,  where  they  are  converted  into  the  two 
strata  of  retinal  pigment  layer  of  the  iris  (pars  iridis  retinae). 

The  blood  supj^ly  of  the  ciliary  is  derived  from  the  long  pos- 
terior and  the  anterior  ciliary  arteries;  these  arteries  break 
into  numerous  twigs  which  pass  to  the  veins.  The  larger 
\eins  pass  backvvar<l  to  the  \  enac  xorticosac ;  others  pierce  the 
sclera,  ccjming  into  \  iew  under  the  coniuncti\  a.  near  the  corneal 
ring,  and  anastomose  with  the  conjunctival  veins. 

Ciliariscope.     An  instninicnt   for  ix.inuning  the  liliary  regiim  of 
the  eye. 

Ciliosis.     A  nervuus  t\viti.liing  of  the  cvelul. 


CIRCLES  OF  DISPERSION  95 

Circles  of  Dispersion.  Figures  produced  on  the  retina  by  di- 
vergent and  convergent  light-waves  which  enter  the  eye  wheft 
it  is  adjusted  for  neutral  waves.  Such  waves,  of  course,  are 
not  represented  on  the  retina  by  focal  points,  but  by  circles. 
These  circles  of  dispersion  take  the  form  of  the  pupil. 

Circles  of  Haller.     \'ascular  circles  in  the  eye. 

Circumocular.     Surrounding  the  eye. 

Circum-Orbital.     Surrounding  the  orbit. 

Clinoscope.  An  instrument,  devised  by  Stevens,  for  determining 
the  declination  of  the  vertical  meridians  of  the  eye.  Although 
primarily  an  instrument  of  research,  it  is  of  some  practical 
value  in  cases  of  paralysis  of  ocular  muscles  and  in  anomalous 
adjustment  of  the  eyes. 

Clonic.  A  term  applied  to  muscular  spasmodic  contractions 
which  alternate  with  periods  of  relaxation,  in  distinction  from 
tonic  spasms  which  persist  continuously. 

Cocaine.  Alkaloid  of  coca  leaves.  A  drug  wdiich  produces  local 
anesthesia,  by  paralyzing  peripheral  nerve  endings.  It  is  much 
used  in  eye  work  in  a  4%  to  10%  solution.  Incidentally,  it 
dilates  the  pupil  by  paralyzing  the  ciliary  nerves,  but  its  effect 
is  transient,  and  it  is  not  used  for  this  purpose. 

Coddington  Lens.  A  lens  used  for  magnifying  the  anterior  tis- 
sues of  the  eye,  the  cornea,  etc.,  for  examination. 

Collargol.    A  colloid  form  of  silver  salt,  used  as  a  collyrium. 

Collimate.    To  bring  the  axes  of  a  binocular  instrument  into  line. 

Collimator.  A  part  of  a  binocular  instrument  which  brings  the 
axes  into  line. 

Collinear.     Lying  in  the  same  straight  line. 

Collinear  Space-Systems.  The  name  given  to  a  point  to  point 
system  of  uniting  object-space  and  image-space  by  means  of 
rectilinear  rays.  (See  Aplanatic.)  It  is  also  known  by  the 
term  CoUineation,  introduced  by  Moebius. 


96  COLLOID 

Colloid.  A  substance  which  in  solution  does  not  pass  easily 
through  a  semi-permeable  meml)rane. 

Collyrium.  A  medicine  to  be  used  in  the  eye  in  the  form  of 
drops. 

Colmascope.  An  instrument  for  showing  strains  in  lens  blanks. 
Uy  transmission  of  powerful  light  the  strain  is  shown  by 
shadows  and  interference  colors. 

Coloboma.  An  absence,  or  a  break  in  the  continuity,  of  one  of 
the  vascular  tissues  of  the  eye,  usually  of  the  iris  •  or  the 
chorioid.  Colobomata  may  be  congenital,  or  acquired  through 
injury  or  disease,  or  artificially  produced  by  operation.  Those 
of  the  iris  are,  of  course,  visible  to  the  naked  eye,  the  black 
pupil  showing  through  the  break.  Those  of  the  chorioid  are 
seen  with  the  ophthalmoscope  as  patches  of  white  showing 
through  the  retina. 

Color.  This  word,  like  the  word  "light,"  has  two  applications: 
a  physical  or  objecti\e  application,  and  a  physiological  or  sub- 
jective one.  In  the  first  use  of  the  word  wc  refer  to  the  physical 
phenomena  which  are  responsible  for  the  production  t)f  color; 
in  the  second,  to  the  impressions  or  sensations  jjroduced  on 
the  brain  by  the  action  of  tliese  phenomena  upon  the  retina  of 
the  eye.  It  is  to  be  noted,  however,  that  the  names  and  ciual- 
ities  which  we  ascril)e  to  the  physical  i)henonK'na  are  deri\ed 
from,  and  determined  by,  the  subjective  sensations  which  they 
produce. 

(1)  Piiysically,  C(jlor  finds  its  e(|uivalent  in  the  number  of 
oscillations  j^er  second  of  the  various  ligiit  waves;  or.  what  is 
the  same  thing,  the  lengths  (jf  these  various  waves  from  crest 
to  crest.  Naturally,  since  color  is,  in  the  last  analysis,  a  sub- 
jective sensation,  fcjrming  an  integral  part  of  vision,  only  those 
light  waves  which  fall  within  the  range  of  retinal  reaction, 
i.  e.,  only  those  which  stimulate  the  sensation  of  \isittn,  can 
be  regarded  as  being  color  waves. 

It  has  been  found  by  e.xperiment  that  the  slowest  and  long- 
est of  these  visual  light  waves  give  the  sensatii)n  whicii  we 
call  red;  the  fastest  and  shortest  give  the  sensation  of  violet. 
Slower,  longer  waves  than  n-d   (infra-red)  and  faster,  shorter 


COLOR  97 

waves  than  violet  (ultra-violet),  give  no  color  sensation  at 
all.  About  midway,  as  to  wave-length,  between  red  and  violet 
are  waves  which  produce  the  sensation  of  green  ;  and  inter- 
mediately between  these  three  are  gradations  of  wave-lengths 
which  give  a  series  of  color  sensations  in  which  the  red  ele- 
ment gradually  loses  predominance  and  violet  gradually 
assumes  it. 

All  the  different  color  waves  are  contained  in  a  full  beam  of 
white  light,  so  that  it  is  customary  to  say  that  the  sum  of  all 
the  colors  is  white.  This  statement,  however,  can  only  be 
properly  made  from  a  subjective  standpoint.  Experiment  has 
shown  that  each  of  the  color  waves  is  differently  susceptible 
of  refraction  by  a  refracting  medium  ;  the  violet  waves  being 
affected  the  most,  the  red  the  least,  and  so  on,  proportionately 
up  or  down  the  scale.  This  property  makes  it  possible,  by 
means  of  a  refracting  prism,  to  separate  a  beam  of  white  light 
into  its  constituent  color  waves  so  that  they  fall  upon  a  screen 
in  a  continuous  row  of  colors,  merging  into  each  other,  with 
violet  at  one  end,  red  at  the  other,  green  in  the  middle,  and 
the  other  colors  in  their  respective  orders.  The  same  effect  can 
also  be  produced,  in  even  finer  differentiation,  by  diffraction 
through  a  narrow  slit  in  an  opaque  sheet  of  metal.  The  series 
of  colors  thus  separated  out  is  technically  known  as  the  spec- 
trum ;  the  instrument  by  which  they  are  separated,  the  spec- 
troscope. 

The  shorter  the  spectrum,  i.  e.,  the  less  space  into  which 
the  colors  are  drawn  out,  the  fewer  gradations  of  color  sen- 
sations made  upon  the  eye ;  in  other  words,  the  fewer  colors 
seen.  In  a  short  spectrum,  looked  at  as  a  whole,  there  seem 
to  be  but  four  colors,  red,  green,  blue  and  violet,  because,  by 
contrast  with  these  decided  colors  the  short  transition  colors 
disappear.  When  the  spectrum  is  made  longer,  the  transition 
colors  are  better  recognized  ;  although  at  no  time  does  any 
color  make  its  full  impression  on  the  eye  while  other  colors 
affect  it  simultaneously.  By  means  of  an  ordinarily  long 
spectrum  we  are  able  to  distinguish  eight  general  color  sensa- 
tions, which,  with  their  rates  of  oscillation  and  wave-lengths, 
are  as  follows : 


98 


COLOR 


V.^f/       BUc      Ctf*"*.     "*«ll'>~   Ot«^«  ffi 
hUttyati\<^  Bi'^tniOh.   of   Color    Wi^ei   Uj    P  r  <  i  m. 

Millions  of      Wave  Length 
Millions  of  \'ibra-  in 

tions  per  second,  micrometers 

Extreme  red   395  760 

Red   -^^7  686 

Limit  of  red  and  orange 4.i8  656 

Golden  yellow    510  589 

Green    570  526 

Cyanean  blue 618  486 

Limit  of  indigo  and  violet 697  430 

Violet   757  396 

(The  above  table  is  compiled  frtjiii  Iklnihi)!/.  and  Abney.) 

COMPLEMENTARY  COLORS. 

That  there  is  a  real  dilVcroncc  between  color  as  a  physical 
l)henomenon  and  color  as  a  i)hysi()logical  sensation  is  shown 
in  tlie  fact  thai  tlu-  falling,  simultaneously,  upon  the  retina  of 
two  or  more  certain  c(dor  waves  jjroduces  at  least  two  colors 
which  do  not  exist  in  nature,  namely,  ])urple  and  white.  There 
is  no  part  of  the  spectrum  in  which  the  wa\e-len_i;th  is  such 
that  the  corresponding  ccjlor  would  be  purple;  it  is  a  color 
sensation  only,  ])ro(luced  by  the  simultaneous  impressions  of 
blue  and  red.  The  same  is  true  of  the  color  sensation  of  white, 
produced  by  cond)iuiug  the  effect  of  all  the  ccdors  in  the 
spectrum. 

It  is  jjossible,  liowe\er,  to  obl.iiu  white  from  cond)iu;itions 
(jf  tW(i  simjile  ciihjrs  taken  froui  differeut  parts  of  the  si)eclrum 


COLOR  99 

and  mixed  in  certain  ratios.     Two  such  colors  are  called  com- 
plementary, a  series  of  which  is  here  given  from  Helmholz : 
Color.  Wave  Length.  Color.  Wave  Length. 

Red    656  Green-Blue    ....  492 

Orange    607  Blue   489 

Golden  Yellow.  .   585  Blue   485 

Yellow    567  Indigo-Blue 464 

Green-Yellow    ..   563  Violet    433 

Green  has  no  simple  complementary  color,  but  only  a  com- 
pound  one — red  and  blue,   or  purple. 

It  should  be  borne  in  mind  that  what  is  here  stated  con- 
cerning colors  relates  only  to  pure  colors  of  the  solar  spec- 
trum, i.  e.,  those  which  reside  in  a  pure  beam  of  white  sun- 
light. As  a  matter  of  fact  the  colors  in  nature  with  which  we 
have  most  to  do — such  as  the  colors  of  flowers,  minerals,  etc. 
— are  almost  never  pure  colors,  and  therefore  are  subject  to 
variations  from  the  behavior  here  attributed  to  the  pure  colors. 
When  we  come  to  deal  with  artificial  pigments,  we  are  con- 
fronted with  even  greater  complications,  due  not  only  to  the 
lack  of  purity  in  the  colors,  but  also  to  the  absorption  and  re- 
flection which  these  pigments  exercise  upon  the  different  color 
waves. 

COLOR  PROPERTIES. 
Colors  have  three  elements  of  quality,  (1)  hue,  (2)  purity, 
and  (3)  brightness.  The  first  is  its  true  color  quality,  giving 
the  sensation  of  red,  blue,  etc.,  and  depends  upon  the  length 
and  velocity  of  the  wave ;  it  corresponds  to  pitch  in  sound. 
The  second  (often  called  saturation)  depends  upon  the  admix- 
ture of  white ;  the  less  white  mixed  with  it,  the  purer  the  color, 
or  the  greater  the  saturation.  The  third  quality,  sometimes 
termed  luminosity,  depends  upon  the  vigor  of  the  ether  move- 
ment which  it  produces,  and  is  therefore  proportional  to  the 
square  of  the  greatest  velocity  of  the  ether  particles.  Sub- 
jectively, of  course,  a  color  may  be  brighter  according  to  the 
sensitiveness  of  the  retina  to  that  particular  color. 

(2)  The  subjective,  or  physiological  side  of  color  is  not  as 
well  understood  as  its  physical  side,  and  is  in  much  dispute. 
It  is  generally  assumed  that  the  rods  and  cones  of  the  retina 
contain  different  kinds  of  photo-chemical  substances,  each  of 


100  COLOR 

which  reacts  to  a  different  wave-length  of  light,  thus  mediating 
the  sensation  of  a  different  color.  As  to  the  details  of  the 
process  there  are  several  different  theories. 

The  Young-Helmholz  theory,  originated  by  Thomas  Young 
and  elaborated  by  Helmholz,  holds  that  there  are  three  such 
retinal  substances,  reacting  most  strongly  to  red,  green  and 
blue  wave-lengths,  respectively,  although  each  of  them  is 
slightly  aft'ected  by  all  other  waves.  Single  stimulation  by 
light-waves  of  either  of  these  substances,  therefore,  produces 
the  single  sensation  of  red,  green  or  blue,  as  the  case  may  be. 
All  other  color  sensations  are  produced  by  mixed  stimulation 
of  two  or  more  substances  by  two  or  more  wave-lengths,  in 
varying  degrees  of  intensity.    Thus : 

Red  stimulates  strongly  the  red-sensitive  substance,  the 
green  less,  and  the  l)lue  least  of  all.     Sensation  eciuals  red. 

Yellow  excites  moderately  the  red  and  the  green-sensitive 
substances,  the  blue  but  little.     Sensation  equals  yellow. 

Simple  violet  excites  strongly  the  blue-sensitive  substance, 
the  green  less,  and  the  red  less.     Sensation  equals  violet. 

The  Hering  theory,  of  more  recent  date,  also  assumes  three 
photo-chemical  substances,  but  ascribes  to  each  a  two-fold 
action,  namely,  that  under  the  influence  of  certain  wave- 
lengths they  undergo  anabolic  (l)uilding-up)  changes,  and 
under  certain  other  wave-lengths  catabt)lic  (tearing-down) 
changes.  The  double  color-sensation  thus  ascribed  to  the  three 
substances  are  as  follows: 

White  —  r.lack 
Red  —  Green 
Yellow  —  Blue 

(Jlher  color  sensations,  under  the  liering.  as  uinler  the  llelin- 
holz  theory,  are  due  to  mixed  stimulation. 

The  three  color  sensations  of  single  stimulation  predicated 
by  the  Young-Ilelmholz,  and  the  six  predicated  by  liering, 
theories,  are  known  as  primary  color  sensations.  Mixed  sen- 
sations are  called  secondary  color  sensations. 

COMPLEMENTARY  COLORS. 

It  has  already  been  said,  wluii  treatiiij^  of  tin-  physical  aspect 
of  the  subject,  that  it  is  i)ossil)lr,  by  combining  certain  pairs 
of  colors  of  the   si)ectruni,  to  i)roduce  a   sensation   of   white; 


COLOR  101 

and  that  these  pairs  were  known  as  complementary  colors. 
Subjectively,  it  is  also  to  be  observed  that  these  complementary 
colors  are  mutually  exclusive  of  each  other  in  their  excitation 
of  color  sensation ;  that  is  to  say,  if  both  wave-lengths  fall  on 
the  retina  simultaneously,  it  is  impossible  for  the  brain  to 
perceive  both  of  them.  If  one  stimulation  is  in  excess  of  the 
other,  the  brain  will  perceive  the  color  whose  stfmulation  is  in 
excess,  and  not  the  other  at  all ;  if  the  two  stimulations  are 
equal  in  intensity,  then  each  offsets  the  other,  and  the  brain 
perceives  no  color  at  all,  but  white. 

It  must  be  confessed  that  the  Young-Helmholz  theory, 
which  at  present  is  the  most  generally  held  among  physiolo- 
gists, offers  very  scant  explanation  of  this  phenomenon  of 
complementary  and  contrast  colors.  Hering,  on  the  contrary, 
built  his  theory  up  implicitly  out  of  a  consideration  of  the  be- 
havior of  these  colors.  His  three  primary-color  pairs  com- 
prise the  fundamental  contrast  and  complementary  colors.  If 
either  one  of  these  photo-chemical  substances  be  stimulated 
simultaneously  b}-  the  two  wave-lengths  to  which  it  reacts,  the 
nature  of  tlie  result  will  depend  upon  the  comparative  intensity 
of  the  two  stimulations.  If  the  anabolic  stimulation  is  in  ex- 
cess, the  reaction  will  be  an  anabolic  one,  and  the  color  sensa- 
tion will  be  that  of  the  anabolic  color;  if  the  catabolic  stimula- 
tion is  in  excess,  the  reaction  will  be  catabolic,  and  the  opposite 
color  sensation  result ;  if  the  two  stimulations  be  equal,  anab- 
olism  and  catabolism  will  balance  each  other,  no  color  sensa- 
tion will  result,  and  neutral  white  will  be  seen.  But  the  sub- 
stance cannot  undergo  both  anabolic  and  catabolic  changes  at 
one  and  the  same  time ;  hence  it  is  impossible  to  perceive  both 
of  the  two  contrast  colors  at  once. 

Subjectively,  again,  these  complementary  colors  express 
themselves  in  characteristic  after-images.  If  the  eye  gaze 
steadily  for  several  seconds  upon  a  primary  color,  especially 
in  great  saturation,  and  then  either  be  closed,  or  else  be  fixed 
upon  an  area  of  neutral  tint,  an  area  of  complementary  color 
will  be  seen  similar  in  size  and  shape  to  that  of  the  primary 
color  first  looked  at.  Thus,  if  one  first  looks  at  red,  the  after- 
color  will  be  green;  if  blue,  then  yellow. 


102  COLOR 

Plelmholz  explains  this  on  the  ground  that  the  photo- 
chemical substances  of  the  retina  are  fatigued  by  first  view- 
ing the  piimary  color,  so  that  when  the  eye  is  directed  to  neu- 
trality it  sees  only  a  combination  of  the  other  two.  Thus,  by 
looking  at  red,  the  red-sensitive  substance  is  temporarily  ex- 
hausted, so  that  as  an  after-color  the  eye  can  only  see  green 
and  blue.  Hering's  explanation  is  that  the  after-color  sensa- 
tion is  produced  by  the  building  up  of  the  substance  which  had 
been  previously  disintegrated,  or  vice  versa. 

It  is  known,  in  a  general  way,  that  the  various  color  waves, 
if  made  to  fall  continuously  for  long  periods  of  time  upon  the 
eye,  produce  characteristically  different  physical  effects  upon 
the  tissues  of  the  eye,  not  only  the  retina,  but  other  parts.  It 
is  a  subject,  however,  which  has  not  been  very  systematically 
studied,  and  has  yet  to  be  investigated.  We  have  learned  the 
benefits  of  keeping  out  of  the  eye,  in  certain  cases,  the  actinic 
waves  (ultra-violet),  by  means  of  certain  qualities  of  lenses; 
but  differentiation  between  the  color  waves  themselves  has  not 
as  yet  been  carried  out  to  any  great  extent.  Although  num- 
erous devices  have  been  made  to  test  the  sensitiveness  of  the 
eye  to  different  colors,  no  very  practical  applicatipn  of  them  has 
yet  been  made. 

Color  Adaptation.  The  adajjtation  of  the  retina  for  the  percep- 
tion of  colors  on  going  from  ;i  relatixely  light  room  into  a 
relatively  dark  one,  or  the  rexerse.  The  subject  has  been  ex- 
tensi\ely  imestigated  by  indridge-Cireen. 

Color-Blindness.  This  condition  was  discoxered  and  demon- 
strated by  Oalton,  an  English  physicist,  who  was  himself  a 
victim  of  it,  and  is  sometimes  called,  after  him.  Daltonism.  It 
consists  in  an  inability  to  distinguish  any,  or  certain,  of  the  pri- 
mary colors  of  the  spectrum.  In  the  first  case  it  is  said  to  be 
total  color-blijidness ;  such  i)eople  (very  rare)  live  their  lives 
in  a  world  of  bl;ick-and-white  and  gray.  In  the  second  case  it 
is  c.illed  p.irlial  color-blindness:  and  persons  of  this  type  are 
much  more  numerous  tli.m  w  ;is  formerly  supposed.  .According 
to  the  N'oung-ilclmhol/  theory  of  color  perception  (see  Color) 
we  designate  the  pci  son  who  recognizes  only  two  primary 
colors  a   (lichroniati- ;   one    who    recognizes   bul    one    .-i    mono- 


COLOR 


103 


chromate.  A  normal  person  is  a  trichromate.  These  terms 
would  hardly  apply  under  the  Hering  theory. 

The  condition  is  generally  supposed  to  be  due  to  a  lack  in  the 
retina  of  the  chemical  substance  or  substances  which  react  to 
the  stimulation  of  the  particular  color-waves  to  which  there  is 
blindness.  Naturally,  blindness  to  one  color  involves  blindness 
to  its  complementary  color,  according  to  the  theories  of  both 
Helmholz  and  Hering.  And,  in  general,  this  is  what  we 
actually  find  in  practice.  The  commonest  form  is  blindness 
for  red  and  green.  However,  there  are  certain  findings  in 
some  cases  of  color-blindness  which  are  not  satisfactorily  ex- 
plained by  either  theory ;  and,  in  fact,  no  theory  of  color  per- 
ception has  yet  been  ofifered  which  accounts  for  all  the  normal 
and  pathological  phenomena. 

In  the  commonest  forms  of  color-blindness  the  two  colors 
that  are  seen  are  yellow  and  blue,  between  which,  in  the  spec- 
trum, is  a  neutral  grey  area.  Of  such  color-blindness  there 
are  two  general  types,  (1)  the  so-called  red-blindness,  where 
the  spectrum  is  seen  shortened,  at  the  red  end,  the  shades  of 
red  appearing  as  dark  areas,  and  (2)  the  so-called  green-blind- 
ness, where  the  spectrum  does  not  appear  shortened.  Under 
the  Hering  doctrine,  both  are  classified  as  red-green  blindness. 

TESTS. 

For  the  scientific  testing  of  color-blindness  a  spectroscope 
is  necessary,  to  determine  whether  or  not  the  spectrum  is 
shortened  at  the  red  end  and,  also,  by  isolating  the  bands  of 
color,  to  discover  how  the  patient  regards  and  compares  the 
single  colors. 


Holmgren's  Color  Test. 


104  COLOR 

In  1875  Holmgren,  of  Sweden,  introduced  his  well-known 
worsted  test,  based  on  a  study  of  color-blindness  with  the  spec- 
troscope. Worsted  was  selected,  first,  because  it  has  no  gloss, 
and  color-blind  persons  are  not  infre(|ucntly  able  to  pick  out 
colors  by  means  of  their  characteristic  glare,  second,  because 
in  flat  colors  one  can  obtain  a  great  degree  of  saturation. 

The  principle  of  the  Holmgren  test  is  that  red-  and  green- 
blind  persons  see  in  the  spectrum  only  two  colors — yellow  and 
blue,  with  a  neutral  gray  band  between  them.  Holmgren  se- 
lected green  as  his  first  test-color  because  it  is  the  whitest 
color  in  the  spectrum  and  corresponds  in  tint  to  the  neutral 
zone,  thus  making  an  excellent  confusion  color  with  pale  shades 
of  gray,  brown,  fawn  and  yellow.  His  second  test  color  is  rose, 
a  mixture  of  red  and  blue,  in  which,  of  course,  the  red-blind  see 
only  blue.  His  third  is  red,  which  aflfords  an  excellent  con- 
fusion with  dark  shades  of  gray,  brown,  etc. 

The  technique  of  the  Holmgren  test  is  as  follows:  The  test 
set  is  to  include  three  large  test  skeins,  green,  rose  and  red,  and 
more  than  a  hundred  small  skeins  consisting  of  red,  orange,  yel- 
low, yellow-green,  pure  green,  blue-green,  blue,  violet,  purple, 
pink,  brown  and  gray,  with  several  shades  and  tints  of  each 
color. 

Test  1.  Ask  the  patient  to  select  from  the  entire  collection 
(placed  in  good  daylight)  all  the  colors  which,  in  general  hue, 
seem  to  him  to  be  like  the  large  green  skein.  The  completely 
color-blind  will  select,  with  or  without  greens,  some  confusion 
colors — grays,  fawns,  pinks,  \ellows,  etc.  The  incompletely 
color-blind  will  match  it  with  greens,  to  which  they  will  add  a 
few  light  shades  of  fawn  or  graw  This  test  indicates  whether 
or  not  the  candidate  is  color-blind — completely  or  incompletely. 
For  further  iintstigation  we  emi)l(iy  another  test. 

Test  2.  Mix  the  colors  up  again,  ami  ask  the  candidate  to 
match  the  rose  skein.  The  color-i)lin(l  will  select  always  the 
light  or  dark  shades  of  blue  and  xioUt.  The  completidy  color- 
blind will  choose  blue  or  \iolet.  with  or  without  purple:  the 
completely  green-blind,  green  or  gray,  with  or  without  purple. 
A  ])atient  proven  color-blind  by  Test  1  is  only  incoinpUtely  so 
if  he  matches  rose  with  deeper  pinples  alone. 


COLOR 


105 


Test  3.  This  is  a  supplementary  test  for  separating  out  the 
reds,  by  having  the  candidate  match  the  red  skein.  The  red- 
blind  will  select,  beside  reds,  shades  darker  than  red ;  the  green- 
blind,  green  and  brown  shades  lighter  than  red.  Only  mark- 
edly color-blind  persons  fall  down  on  this  test. 

The  Jennings  Self-Recording  Test  is  a  modification  of  the 
Holmgren,  devised  by  J.  Ellis  Jennings,  of  St.  Louis,  Mo.  It 
consists  of  a  square  box,  divided  into  two  compartments,  one 
for  the  Green  test  and  the  other  for  the  Rose  test.    The  stand- 


Jennings'  Self-Recording  Test. 

ard  test  skeins  of  green  and  rose,  respectively,  are  attached  to 
the  inside  of  the  box-lid.  In  each  side  of  the  box  is  a  color- 
board  made  up  of  green  and  all  the  green-confusion  colors,  on 
one  side,  and  of  rose  and  all  the  rose-confusion  colors  on  the 
other  side.  In  the  center  of  each  color-patch  is  a  perforated 
hole,  through  which  the  candidate  thrusts  a  stylus  to  register 
his  selection  of  a  match-color.  Beneath  the  color-board  is  a 
record-sheet,  divided  into  squares  corresponding  to  the  color- 
patches,  and  marked  with  a  G  and  an  R,  respectively,  in  those 
squares  which  coincide  with  a  color-patch  which  matches  the 
green  or  the  rose. 

If  the  candidate  be  normal,  there  will  be  a  punch-mark  on 
the  record-sheet  in  every  space  marked  G  and  R.  Any  punch 
mark  in  a  blank  space  indicates  a  mistake.  If  the  mistake  is 
on  a  horizontal  line  with  the  letter  G,  the  mistake  was  made 
in  the  green  test  if  horizontal  with  the  letter  R,  in  the  rose 
test. 


106 


COLOR 


Williams    Lantern. 


jp^^a  Williams'  Lantern  Test.  The  lantern  test 

M/fff///l^  differs  from,  and  supplements,  the  worsted 

l^^^^dB  tost  in  that  (a)  it  tests  the  color  perception 

)H•lV■L*-^..  in  the  central  retinal  area,  where  light  from 
a  distant  lantern  focusses,  and  (b)  it  de- 
termines the  ability  of  the  candidate  to 
recoj^nize  and  name  the  colors  of  signals 
which  he  will  have  to  use  at  night.  In  the 
worsted  test  no  names  are  used  ;  only  com- 
parison of  colors.  It  is.  however,  import- 
ant that  the  candidate  (if  he  be  a  man  using 
signals)  be  able  to  give  to  color  sensations 
the  names  which  normal  persons  give  to 
them. 

The  lanterns,  screened  by  shutters,  are 
lighted  in  a  darkened  room,  and  the  colored 
glasses  made  to  face  him  directly.  By  means  of  revolving  shut- 
ters, the  colored  lights  are  then  revealed  to  him,  usually  two 
or  three  at  a  time,  in  a  sequence  corresponding  to  the  standard 
record  form  used,  and  he  is  required  to  call  out  their  names. 
Where  he  gives  the  name  of  the  color  correctly,  the  examiner 
writes  an  O,  or  O.  K.,  against  it  on  the  record ;  where  he  names 
it  wrongly,  he  writes  in  the  name  which  the  candidate  gives. 
In  this  way  he  is  able  to  interpret  the  completed  record  sheet. 
A  candidate  should  be  rejected : 

If  he  calls  a  red  light  green  or  white ; 
If  he  calls  a  green  light  red  or  white ; 
If  he  calls  a  white  light  red  or  green. 
Bearing  in  mind,  as  stated  above,  that  candidates  are  some- 
times able  to  dilTerentiate  colors  by  their  luminosity,  the  lan- 
tern test  should  pro\ide  for  \aryiiig  the  intensity  of  tlu'  lights. 
so  as  to  remf)\e  this  possible  source  of  malingering. 

W^hen  cobalt  blue  glass  is  used  in  the  test,  it  is  to  bi'  rcinein- 
bcrccl  that  liypfropic  (■.iiididatcs  will  sec  this  color  as  a  center 
of  blue  with  a  red  center;  myopic  i  aiidid.itcs  as  a  center  of 
red  with  a  blue  margin.     (.Sec  Chromatic  Test.) 

In  iiiakiiii;  (be  laiitcin  tt'st  it  is  important  that  tlir  cx.iininer 
make  no  remarks  wliich  will  iiidicati'  wlutlu'i  tlu-  candidate's 
answer  is  right  or  wrong. 


COLOR  CURVES,  MAXWELL'S 


107 


Nagel's  Test  consists  of  a  set  of  cards,  each  bearing  a  series 
of  little  color  disks  arranged  in  a  ring.  In  some  rings  the  disks 
are  all  of  the  same  color  but  in  different  shades ;  in  others 
there  are  two  or  three  different  colors.  By  asking  the  patient 
to  indicate  which  are  monochromatic,  which  dichromatic,  and 
which  trichromatic,  we  readily  ascertain  the  existence  and 
nature  of  his  color-blindness. 

The  Stilling  Plates  consist  of  patterns  set  on  a  back-ground 
of  different  color,  the  tints  being  svich  that  a  color-blind  person 
cannot  distinguish  the  pattern  from  the  back-ground. 

Eldridge-Green,  of  London,  has  also  devised  a  system  of  lan- 
tern tests.  The  colors  yellow,  pure  green,  signal  green,  blue, 
purple  and  red  are  mounted  on  three  discs ;  a  fourth  is  ground 
glass  with  white  light.  The  discs  are  rotated,  being  brought 
before  the  lamp  in  succession,  or  any  desired  combination 
formed.  A  diaphragm  imitates  the  representation  of  railway 
signals. 

Color  Curves,  Maxwell's.     Three  curves  representing  the  three 
primary  colors,  whose  points  of  intersection  with  the  vertical 

G 


♦  0,?  R 


V  G 

^laxwell's   Color   Curves. 


represent  the  relative  amounts  of  each  color  needed  to  produce 
the  required  mixture. 

Color  Mixture.  This  term  has  two  meanings,  (1)  By  the 
physicist  it  is  used  to  designate  the  simultaneous  presentation 
to  the  brain  of  two  color  sensations,  (2)  By  the  painter  it  is 


108 


COLORED  SHADOWS 


employed  to  indicate  tlie  resultant  cijlur  effect  of  mixing  two 
or  more  pigments. 

Colored  Shadows.  Paradoxical  shadows  cast  by  an  opaque  body 
on  a  neutral  screen  from  two  equi-distant  lights  of  equal  in- 
tensity, one  being  white  and  the  other  colored.  The  shadow 
cast  by  the  white  light  appears  to  be  of  the  color  of  the  colored 
light,  while  that  made  by  the  colored  light  aj)pears  to  be  of 
the  complementary  color.  The  eflfect  is  due  to  the  contrasts 
made  by  the  shadows  with  the  mixed  light. 

Color  Table.  A  diagrammatic  figure  constructed  to  represent  in 
graphic  form  the  result  of  mixing  primary  colors.  There  are 
two  such  tables  in  general  use,  namely,  Maxwell's  and  New- 
ton's. 

Creen  , 


Blui5b-Gre<a 


.M;ixu(ir.s   (.'olor  TiibUv 


Maxwell's  table  is  in  the  form  of  a  roimded  triangle.  By 
mixing  two  colors  on  tin-  same  leg  of  the  triangle  a  color  is 
obtained  of  the  same  purity  as  the  spectral  colors;  b\  combin- 
ing tlujse  on  oppositi'  lej^s.  we  get  a  color  mixed  with  white. 


COLOR,  UNITARY  109 

The  angles  of  the  triangle  represent,  respectively,  red,  green 
and  blue. 

Newton's  table  is  a  circle  with  a  triangle  contained  in  it, 
whose  three  angles  also  represent,  respectively,  red,  green  and 
blue.  All  the  colors  producible  by  mixing  two  colors  are  shown 
on  a  straight  line  joining  the  two,  proportionately  nearer  to  one 


yriVowi*b-Cr«eii 


Blnish-Green 


Newton's   Color  Table. 

of  the  colors  as  that  color  dominates  the  mixture.  Since  white 
is  at  the  center  of  the  circle,  any  one  of  the  colors  can  be  joined 
to  white  and  projected  through  the  line  of  the  red-green-blue 
triangle,  thus  indicating  the  formation  of  all  existing  hues. 

Color,  Unitary.  A  color  in  which  it  is  impossible  to  detect  any 
other  color. 

Commissure,  Optic.    The  same  as  the  Optic  Chiasm,  q.  v. 

Commotio  Retinae.  A  functional  disturbance  of  the  retina  due 
to  violence,  such  as  a  blow  on  the  head  or  eye. 

Compound.  In  optics  this  term  is  applied  to  a  reflecting  or  re- 
fracting system,  made  up  of  a  series  of  lenses  or  mirrors,  so 
placed  in  relation  to  each  other  that  their  resultant  action  may 
produce  the  required  effect.  All  modern  telescopes  and  micro- 
scopes are  compound. 


no  coMus 

Compound  Astigmatism  is  astig:matism  in  which  both  the 
chief  meridians  of  the  eye  have  their  ])rincipal  focal  point 
either  behind  or  in  front  of  the  retina.     (See  Astigmatism.) 

Compound  Lens.  A  lens  containing  both  a  sphere  and  a 
cylinder.     (See  Lens.) 

Comus.  A  crescentic  patch  of  yellow  near  the  optic  disc  seen 
in  high  myopes.  It  is  due  to  degenerated  chorioid  showing 
through  the  retina,  caused  by  stretching  of  the  chorioid. 

Concave.  Hollow  and  curved,  as  the  inner  surface  of  a  sphere. 
Mathematically,  a  surface  is  concave  when  a  straight  line 
drawn  from  point  to  point  falls  between  the  surface  and  the 
eye.     (See  Lens.) 

Concavo-Convex.  Concave  on  one  side  and  convex  on  the  other. 
(See  Lens.) 

Concentric.  Developed  about  a  common  center.  Concentric 
lens  surfaces  in  series  exactly  neutralize  each  other's  refract- 
ing efifects. 

Concomitant.     (Also  written  Comitant.)     See  Strabismus. 

Condenser.  A  combination  (»f  two  plano-convex  lenses,  with 
their  convex  surfaces  in  contact,  for  the  jnirpose  of  gathering 
up  aberrant  and  divergent  rays  of  light  and  focussing  them  on 
a  screen.  Used  in  various  kinds  of  image-making  instruments. 
Also  called  a  Condensing  Lens. 

Cone,  Ocular.  TIr-  cone  of  lit^iit  fornu-il  within  the  eye  by  the 
rays  entering  the  i)ui)il  and  focussing  on  the  retina. 

Cone  Test.     See  Imbalance,  Muscular. 

Conical  Cornea.  A  bulj^^ing  of  the  conu-a  forward  in  the  shai)e 
of  a  cone,  due  to  stretching.  Tlu'  cornea  is  always  very  thin 
and  easily  penetrated.  Refraction,  of  course,  is  greatly  dis- 
t(jrtcd,  for  wliicli  tlic-rc  is  no  rrnu'dy  except  the  pin-hole  disc. 
The  condition   belongs  to  the   eye-surgi-oii. 

Conjugate  Foci.  Two  points  in  a  reflecting  or  refracting  system 
so  situated  with  rclati<in  to  r;\{U  other  as  tu  be  o|)tiially  inttr- 


CONJUNCTIVA  111 

changeable.     Conjugate   foci   situated  on   secondary  axes   are 
called  secondary  conjugate  foci. 

Conjunctiva.  The  mucous  membrane  which  lines  the  anterior 
surface  of  the  eyeball  and,  being  reflected,  covers  the  inner 
aspects  of  the  two  eyelids.  That  which  covers  the  eyeball  is 
known  as  the  bulbar,  and  that  which  lines  the  lids  the  pal- 
pebral, conjunctiva.  The  portion  which  lines  the  upper  eyelid 
(conjunctiva  tarsi)  is  adherent  to  the  tarsus,  and  contains 
small  glands,  known  as  the  glands  of  Krause.  This  portion 
derives  its  blood  supply  from  two  arterial  arches,  the  superior 
and  inferior  tarsal  arch.  In  the  lower  lid  there  is  but  one 
arch. 

In  the  fornix,  between  lower  and  upper  lid,  the  conjunctiva 
is  extremely  loose,  so  as  to  permit  of  free  movements  of  the 
eyeball  without  hindrance  from  the  lids. 

The  bulbar  conjunctiva  is  continuous  over  the  cornea,  to 
which  it  is  so  closely  adherent  that  it  may  be  regarded  as  one 
of  the  layers  of  the  cornea  itself.  Over  the  sclera  (conjunc- 
tiva sclerae)  it  is  attached  by  loose  connective  tissue,  which 
is  easil}  undermined  so  as  to  separate  it  from  the  sclera.  There 
are  no  glands  in  the  bulbar  portion.  The  blood  supply  is  de- 
rived from  the  posterior  conjunctival  and  anterior  ciliary 
arteries,  which,  at  the  corneal  ring  suddenly  pass  into  the  in- 
terior of  the  eye,  but  before  doing  so  form  loops  for  the  nutri- 
ment of  the  cornea.  There  are,  of  course,  no  vessels  in  the 
transparent  conjunctiva  that  covers  the  cornea. 

Conjunctivitis..  Inflammation  of  the  conjunctiva.  There  are 
countless  varieties  of  this  trouble,  whose  diagnosis  must  be 
left  to  the  oculist.  Some  varieties  are  comparatively  harmless, 
others  exceedingly  virulent.  All  present  the  ordinary  signs 
of  inflammation  in  the  membrane,  so  that  their  presence  is 
easy  to  recognize. 

The  chief  diagnostic  point  in  conjunctivitis  is  to  distinguish 
it  from  keratitis  and  deep  ciliary  troubles.  This  is  very  readily 
done  by  noting  the  character  of  the  injection  of  the  vessels.  In 
conjunctivitis  this  injection  is  superficial,  brick-red  in  color, 
and  diffused  over  the  eyeball.  The  separate  vessels  are  easily 
distinguished.     (See  Keratitis.) 


112  CONJUNCTIVOPLASTY 

Conjunctivoplasty.     Plastic  operation  on  the  conjunctiva. 

Consensual  Reflex.  A  light  reflex  or  accommodation  in  which 
one  eye  responds  to  stimulation  of  the  other.     See  Reflex. 

Conservation  of  Vision.    See  Hygiene  and  Illumination. 

Contractor  Pupillae.  The  sphinctrc  muscle  of  the  iris  which 
contracts  the  pupil. 

Contra-Generic.  Of  opposite  kinds.  Applied  to  two  surfaces  of 
opposite  curvatures  in  a  compound  lens. 

Contra-Lateral.     ^  )n  (Ji)i)osite  sides,  as  contra-iatcral  blindness. 

Conus.     .See  Staphyloma,  Posterior. 

Convergence.  In  optics  in  general,  the  term  convergence  is  used 
to  describe  two  lines  which  ai)proach  each  other  at  a  vanishing 
angle.  In  physiologic  optics,  the  word  is  specifically  employed 
to  denote  the  turning  of  the  eyes  inward  so  as  to  converge  the 
visual  axes  in  order  to  achieve  sigleness  of  binocular  vision. 
For  this  it  is  necessary  that  light  from  identical  points  on  the 
object  shall  fall  ujKjn  the  yellow  spots  of  the  twt)  eyes  simul- 
taneously. For  this,  again,  the  \isual  axes  must  both  be  di- 
rected toward  the  point  viewed. 

If  the  object  be  situated  at  infinity,  the  \isual  axes  must  be 
practically  parallel;  and  thev  are  practically  ])arallel,  normally, 
when  the  eyes  are  in  a  condition  of  static  balance.  If,  how- 
ever, the  object  be  situated  at  a  point  nearer  than  infinity, 
then,  in  order  that  the  \isual  axes  may  meet  at  a  point  on 
the  object,  the  eyes  must  be  turned  inward.  This  is  accom- 
plished by  means  of  the  internal  rectus  muscles,  and  is  knt)wn 
as  convergence.  The  external  recti  turn  the  eyes  outward, 
restoring  them  to  parallelism,  this  function  bi-ing  known  as 
negative  convergence. 

The  physiological  as])ects  of  con\ergenci'.  and  the  ner\e 
paths  by  which  it  is  mediated,  are  discussed  under  the  section 
devoted  to  Physiology  of  Vision.  We  shall  here  confine  our- 
selves to  a  consideration  of  the  nitiliauical  and  oj)tic;il  phases. 

MATHEMATICS    OF   CONVERGENCE. 

The  mathem.'itics  of  coiuergence  is  that  of  a  triangle  whose 
base  is  a  straight  line  drawn  between  the  centers  of  rotation 


CONVERGENCE  113 

of  the  two  eyeballs — or,  for  working  purposes,  between  the  two 
pupillary  centers — and  whose  sides  are  the  two  converging 
visual  axes.  With  a  normal  pair  of  eyes,  under  normal  con- 
ditions, this  triangle  is  an  isosceles  triangle,  i.  e.,  its  two  sides 
are  equal  and  the  two  angles  made  by  these  sides  with  the 
base  are  equal,  because  the  innervation  of  the  two  eyes  is 
exactly  equal  and  the  degree  of  convergence  made  by  the  two 
visual  axes  similarly  equal. 

The  angle  made  by  the  two  visual  axes  with  each  other  is 
known  as  the  angle  of  convergence.  It  varies,  in  the  same 
individual,  inversely  as  the  distance  for  which  convergence  is 
being  made.  At  the  far  point,  which  in  normality  is  at  infinity, 
the  angle  is  zero.  At  near  point  the  angle  is  greatest.  The 
actual  size  (in  degrees)  of  the  angle  at  any  given  point  varies 
in  different  individuals  according  to  the  length  of  the  base  of 
the  triangle,  i.  e.,  according  to  the  pupillary  width. 

As  many  of  the  practical  problems  of  convergence  have  to 
do  with  abnormal  conditions,  under  which  the  two  visual  axes 
do  not  converge  alike,  this  angle  of  convergence  is  not  of  much 
working  value  in  dealing  with  such  problems. 

THE  METER  ANGLE. 

More  commonly  employed  as  a  unit  of  measurement  is  the 
angle  made  by  each  visual  axis  with  a  median  line  drawn  per- 
pendicularly from  the  middle  of  the  base  line.  This  angle,  at 
1  meter  from  the  eyes,  is  called  the  meter  angle,  commonly 
designated  as  m.  a.  At  other  distances,  nearer  to  and  further 
from  the  eyes,  it  is  a  multiple  or  a  dividend  of  the  meter  angle. 
Distance  and  meter  angle  vary  inversely  with  each  other ;  are, 
in  fact,  reciprocals  of  one  another.  At  50  centimeters  the  angle 
is  equal  to  2  meter  angles ;  at  2  meters  it  is  equal  to  .50  m.  a. 

The  meter  angle,  as  a  unit  of  measurement  for  convergence, 
has  two  advantages:  (1)  It  affords  a  separate  angular  unit  for 
each  eye's  convergence,  and  (2)  it  coincides  at  all  points  with 
the  unit  of  dioptric  measurement  of  accommodation.  At  1 
meter,  normal  accommodation  is  1  D.,  convergence  1  meter 
angle ;  at  50  cm.,  accommodation  is  2  D.,  convergence  2  m.  a. ; 
at  2  meters,  accommodation  is  .50  D.,  convergence  .50  m.  a. 
Nevertheless,  the  meter  angle  is  not  much  employed  in  work-- 


114  CONVERGENCE 

ing  out  practical  problems  of  convergence,  even  in  association 
with  accommodation  problems. 

THE  PRISM  DIOPTRE. 
Much  more  generally  utiH/.ed  than  either  of  the  foregoing 
methods  is  the  prism  dioptre  system  of  measuring  converg- 
ence, which  is,  after  all,  the  more  practically  useful,  because 
most  of  our  clinical  testing  of  muscular  conditions  is  done  by 
means  of  prisms.  The  deviation  made  by  the  eye  in  con- 
vergence is  regarded  as  the  equivalent  of  the  deviation  given 
by  a  prism  to  a  ray  of  light,  and  is  measured  in  the  same  way 
— i.  e.,  1  cm.  of  deviation  in  1  meter  of  distance  constitutes  1 
prism  dioptre  of  convergence.  The  degree  of  convergence,  in 
prism  dioptres,  is  thus  easily  calculated  for  any  point  by  divid- 
ing the  distance  (in  meters)  into  the  deviation  made  by  the 
visual  axis  (in  centimeters). 

At  infinity,  of  course,  the  deviation  of  the  visual  a.xis  is  nil ; 
hence,  also,  the  prism  dioptrism  of  the  convergence  is  zero. 
At  any  other  distance  the  deviation  is  a  constant  quantity. 
namely,  half  the  pupillary  width,  because  the  deviation  is  al- 
ways to  the  median  line.  The  distance  fur  which  convergence 
is  made,  therefore,  (in  meters),  divided  into  half  the  pupillary 
width,  gives  the  prism  dioptres  of  convergence  for  that  point. 
As  the  meter  angle  is  the  reciprocal  uf  the  distance  for  which 
convergence  is  made,  it  is  evident  that  multiplying  the  meter 
angle  of  convergence  by  half  the  ])upillary  width  converts  the 
meter  angle  measurement  into  ])rism  dioptres.  I'urtlu-r,  as  the 
average  pupillary  width  is  a  triile  over  6  cm.,  the  i)rism  dioptres 
are  a  little  over  3  times  the  meter  angles  for  any  given  jioint 
of  convergence.  And,  once  more,  as  the  nu-ttr  angle  coincides 
with  the  n(;rmal  accommodation  for  any  given  point,  it  fol- 
lows that  thf  normal  relation  between  cf)nvergence.  in  prism 
dioptres,  and  accommo(l;ition.  in  dioptres,  is  api>ro\iin;iteiy 
as  three  to  one. 

AMPLITUDE  OF  CONVERGENCE. 

The  pr.ictical  problems  of  con\erj;ence  are  rather  ditti-reut 
from  those  of  accomniod.itioii.  Tlu-  imch.inical  conditions  ;ire 
also  somewhat  diHerent.  In  the  case  of  .accommodation  we 
arc  dealing  with  a  single  muscle  (the  ciliary)  whose  range  of 


CONVERGENCE  115 

function  extends  frankly  from  complete  relaxation  to  maximal 
contraction.  In  convergence  we  are  dealing,  on  the  other 
hand,  with  a  pair  of  muscles  (the  internal  and  external  recti) 
whose  functions  oppose  and  counteract  each  other  at  every 
point. 

The  far  point  of  accommodation  denotes,  and  is  physiologic- 
ally determined  by,  the  complete  relaxation  of  the  ciliary.  For 
any  point  beyond  that  distance,  i.  e.,  for  any  light-waves  of 
greater  convergence,  the  eye  has  no  way  of  adapting  itself.  In 
an  emmetropic  eye  we  say  (although  it  is  not  quite  true)  that 
this  far  point  is  at  infinity ;  that  is  to  say,  the  light-waves  to 
which  it  is  adapted  when  the  ciliary  is  completely  relaxed  are 
neutral  waves ;  waves  of  any  less  curvature  than  neutral  it  has 
no  way  of  focussing  on  its  retina ;  therefore,  for  any  point  be- 
yond infinity  the  emmetropic  eye  has  blurred  vision. 

In  the  case  of  convergence  we  are  confronted  with  a  dif- 
ferent situation.  We  say  (although,  again,  not  ciuite  truth- 
fully) that  with  both  internal  and  external  recti  in  a  state  of 
rest, — or,  better,  in  static  equilibrium — convergence  in  a 
normal  pair  of  eyes  is  adapted  for  infinity,  i.  e.,  for  parallel 
rays.  But  if  a  pencil  of  somewhat  convergent  waves  be  ofifered 
to  the  eyes,  they  still  have  a  way  of  adapting  themselves  to 
them,  by  a  little  extra  innervation  of  the  externals,  turning  the 
eyes  slightly  outward, — known  as  negative  convergence. 

In  determining  the  far  point  of  convergence,  therefore,  it  is 
necessary  not  only  to  find  the  far  point  of  single  binocular 
vision,  but  to  demonstrate  the  far  point  at  which  both  internals 
and  externals  are  in  a  condition  of  static  equilibrium. 

DETERMINATION  OF  THE  FAR  POINT. 

There  are  no  objective  methods  of  ascertaining  the  far  point 
(or  the  near  point)  of  convergence.  Every  test  of  the  powers 
of  convergence  depends  upon  the  patient's  ability  to  maintain 
single  vision.  Fortunately,  its  subjective  manifestation  is  a 
simpler  and  more  definite  affair  than  that  of  accommodation. 
If  a  patient  can  see  singly  a  small  object — a  point  of  light  at 
infinity  or  a  black  dot  at  near  distance — then  it  is  evident  that 
he  is  maintaining  convergence  for  that  distance. 

If,  then,  the  patient  sees  a  single  image  of  a  tiny  round 
light  20  feet  distant,  we  are  sure  that  he  is  adapting  his  con- 


116  CONVERGENCE 

vergence  to  infinity.  The  only  (|uestion  to  be  determined  is 
whether  this  adaptation  is.  as  it  ought  to  be,  a  passive  condi- 
tion, in  virtue  of  "letting  go"  all  active  innervation  of  both  in- 
ternal and  external  muscles,  allowing  the  eyeball  to  assume  a 
position  of  static  equilibrium,  or  whether  it  is  being  maintained 
at  the  expense  of  some  active  muscular  effort,  on  one  side  or  the 
other,  constituting  a  condition  of  unstable  equilibrium. 

The  problem,  then,  resolves  itself  into  one  of  determining 
the  balance  or  imbalance  of  the  two  oi)posing  sets  of  muscles 
when  converging  for  infinity.  If  it  be  found  that  infinity  is 
being  fixed  at  the  expense  of  some  active  effort  on  the  part 
of  the  external  muscles,  then  the  patient's  far  jioint  of  posi- 
tive convergence  is  really  within  infinity  ;  fur  he  is  actually 
exercising  negative  convergence  in  order  to  see  singly.  If,  on 
the  other  hand,  we  find  that  he  is  fixing  infinity  at  the  expense 
of  active  effort  of  his  internals,  then  his  far  point  of  positive 
convergence  is  really  beyond  infinity,  for  he  is  exercising  posi- 
tive convergence  in  order  to  maintain  single  vision  at  infinity. 

DISSOCIATING  THE  IMAGES. 

As  we  have  already  explained  in  the  section  on  Physiology, 
the  maintenance  of  single  binocular  vision  is  incited  by  two 
stimuli,  namely,  the  desire  for  a  clear,  single  image  of  the 
ol)ject  viewed,  and  the  intuitive  impulse  to  make  convergence 
keep  step  with  accommodation.  At  infinity  the  latter  stimulus 
ought  not  to  be  operative.  In  an  emmetrope  it  will  not  be. 
If  it  is  present  it  can  be  eliminated  by  gi\ing  the  i)atient 
full  distance  correction.  The  other  factor, — the  desire  for  a 
single  image — can  also  be  eliminated  if  we  can  deceive  the 
patient  into  the  belief  that  the  images  on  the  two  retinae  repre- 
sent two  diflerent  (jbji'cts ;  he  will  no  ionj^er  desire  to  luse 
them. 

This  is  known  as  "dissociating  tlie  images."  and  is  accom- 
plished by  an  (<i)tical  trick  which  makes  the  two  retin.al  images 
so  dissimilar  in  shape,  size,  color,  etc..  that  the  patient  no 
longer  rej.iar(l->  tlieni  as  liein^  ini.iges  of  the  same  object,  lie 
therefore  makes  no  effort  to  fuse  them,  and  allows  his  eyes  to 
fall  into  their  actual  condition  of  static  eiiiiilibrium.  If  the 
eyes  were  already  in  static  einiilibrinm  lietore  the  iui.i^e.s  were 
dissociated,  then  tlir  two  ini.i^es  reni.iin  tused  ;in\  way.     ll  not. 


CONVERGENCE  117 

as  soon  as  the  eyes,  under  the  influence  of  dissociation,  lapse 
into  equilibrium,  the  two  images  separate — there  is  diplopia — 
and  by  the  direction  in  which  they  separate  we  are  able  to 
judge  which  pair  of  muscles  the  patient  was  actively  exerting 
before  we  tricked  him  into  relaxing  them. 

MADDOX  ROD. 

There  are  several  methods  of  effecting  this  dissociation.  The 
most  commonly  used  is  the  Maddox  rod — a  small  cylinder  of 
red  (or  white)  glass  set  into  an  opaque  disc.  With  this  before 
one  eye,  and  the  other  eye  uncovered,  the  image  of  the  test 
object  (a  small  circle  of  white  light)  is  drawn  out  by  the  rod 
into  a  streak  of  red  light  (at  right  angles  to  the  rod)  in  one 
eye,  while  in  the  other  eye  it  remains  as  a  circle  of  white  light. 
With  no  accommodation  in  force,  and  with  tio  incentive  to  fuse 
the  two  dissimilar  images,  the  eyes  at  once  assume  their  posi- 
tion of  real  static  equilibrium. 

If  the  two  images  still  remain  fused — i.  e.,  if  the  red  streak 
and  the  white  circle  are  seen  in  the  same  lateral  plane — then 
the  patient's  eyes  were  in  static  equilibrium  before  we  dis- 
sociated the  images,  and  infinity  is  his  far  point  of  convergence.^ 

If,  on  dissociation,  the  two  images  separate  in  the  same  di- 
rection as  the  eyes  producing  them,  i.  e.,  the  red  streak  in  the 
direction  of  the  eye  wearing  the  rod,  the  white  circle  in  the 
direction  of  the  uncovered  eye — remembering  that  images  are 
inverted  in  the  eye,  and  seem  to  be,  in  space,  on  the  opposite 
side  to  that  which  they  occupy  on  the  retina — we  know  that 
the  eyes  made  a  turn  inward.  Previous  to  dissociation,  they 
were  being  held  in  fusion  by  an  effort  of  the  external  muscles. 
The  patient's  real  far  point  of  convergence  is  inside  of  infinity. 

If,  on  the  other  hand,  upon  dissociation,  the  images  separate 
in  the  opposite  direction,  (crossed  diplopia),  then  we  know 
that  the  eyes  have  made  a  turn  outward.  They  were  previously 
held  in  fusion  by  an  effort  of  the  internals.  The  patient's  real 
far  point  of  convergence  is  beyond  infinity. 

ESTIMATING  THE  FAR  POINT. 

By  applying  the  prism  dioptre  method  of  measurement,  we 
can  determine  the  precise  amount  that  the  eyes  have  turned, 
inward  or  outward,  under  dissociation.     If  they  have  turned 


118  CONVERGENCE 

inward,  we  have  only  to  find  a  prism  which,  placed  before  the 
eye  with  its  base  out,  will  bend  the  rays  of  light  round  its 
base  just  enough  so  that  the  eyes  will  see  singly  again  ;  and 
that  prism  will  represent  the  turn  that  the  eyes  made  under 
dissociation,  or,  what  is  the  same  thing,  the  effort  of  the  ex- 
ternal muscles  which  they  let  go.  If  the  eyes  made  a  turn 
outward,  we  can  discover  the  same  thing  by  means  of  a  prism 
placed  before  the  eye  base  in. 
According  to  the  formula 

Deviation   (in  centimeters) 
•Prism  dioptre  =  

Distance  (in  meters) 
it  is  a  simple  matter  to  calculate  from  the  finding  where  the 
patient's  far  point  of  convergence  lies.  If  he  proves  to  have 
made  a  turn  of  6  prism  dioptres  inward,  and  his  pupillary 
width  is  6  cm.,  (half  of  this  is  the  deviation),  then  distance 
will  represent  his  far  point : 

3 
p.  r.  =  —  =  .50  cm. 

6 
If  the  turn  had  been  the  same  amount  outward,  then  the  result 
would  be  —  .50  cm.,  or  .50  cm.  beyond  infinity. 

By  the  same  token,  the  prism  dioptres  of  convergence  thus 
determined  at  the  far  point  must  be  subtracted  from  the  prism 
dioptres  at  near  point  of  convergence  in  order  to  determine 
the  amplitude  of  convergence.  Thus,  if  the  patient  showed  6 
prism  dioptres  at  his  far  point,  and  30  prism  dioptres  at  his 
near  point,  his  available  amplitude  is  30  —  6  =  24  prism 
dioptres.  If  he  showed  — 6  prism  dioptres  at  his  far  point  ami 
30  prism  dioptres  at  his  near  point,  then  his  anii)litude  is 
30 — ( — 6),  or  30 -|- 6  ^  36  ])rism  di()i)tres. 

THE  NEAR  POINT. 
'Jlic  (-lctcmiinatii)n  of  tlu-  near  point  of  con\crgencc  does 
not  involve  any  question  of  balance  or  imbalance  between  the 
internal  and  external  nniscles.  It  is  true  that  the  relation, 
normal  or  otherwise,  between  the  two  is  a  determining  factor 
in  the  near  point;  but  the  near  jxtint  contains  and  e.xpresses 
this  factor.  Convergence  at  near  point  is  always  a  condition 
of   unstable   e(|uilibriuMi,   in    the   sense   that    two   antagonistic 


CONVERGENCE  119 

muscles  hold  the  eyes  temporarily  in  a  precarious  poise,  in 
virtue  of  a  powerful  fusion  stimulus ;  and  the  near  point  repre- 
sents the  maximal  net  power  of  the  internals  in  the  struggle. 
We  may,  and  doubtless  shall,  be  interested  in  the  question  of 
balance  at  near  point  for  other  reasons,  but  not  so  far  as  the 
sheer  ascertainment  of  the  near  point  is  concerned. 

The  near  point  of  convergence,  like  its  far  point,  is  demon- 
strable only  by  subjective  means.  There  is  but  one  way  to 
find  it,  and  that  is  to  see  how  close  to  the  eyes  a  small  light  or 
black  dot  can  be  brought  without  the  patient  "seeing  it  double." 
All  prism  measurements  of  positive  convergence  err,  because 
they  eliminate  the  factor  of  accommodation,  which  is  a  legiti- 
mate and  normal  stimulus  of  convergence.  To  place  before 
the  eyes  successively  stronger  prisms,  base  out,  to  discover 
the  strongest  prism  power  with  which  the  patient  can  main- 
tain single  vision  of  an  object  at  infinity,  measures  only  con- 
vergence apart  from  accommodation  ;  and  such  a  test  neither 
can  nor  does  measure  the  normal  amplitude  of  convergence. 
This  form  of  muscle  test  has  its  value ;  but  not  for  determining 
the  near  point  or  amplitude  of  convergence — for  which  the  test 
must  leave  the  interplay  between  accommodation  and  converg- 
ence unhindered. 

The  prism  test  just  referred  to  is  technically  called  the  adduc- 
tion test.  It  seldom  develops  more  than  24  to  25  prism  dioptres 
of  power  in  the  internal  muscles.  Natural  convergence  in  a 
normal  individual  usually  shows  an  amplitude  of  30  to  32  prism 
dioptres. 

ACCOMMODATION  CONVERGENCE  AND  FUSION. 

We  have  already  noted  that  a  certain  ratio  exists  between 
the  accommodation  and  convergence  normally  exercised  for 
any  given  point,  which,  expressed  in  terms  of  meter  angles 
and  dioptres  respectively,  is  a  ratio  of  equality,  and  in  terms 
of  prism  dioptres  and  lens  dioptres  is  as  3  to  1.  We  have  also 
recognized  that  accommodation  is  one  of  the  factors  in  the 
stimulation  of  convergence. 

All  of  the  convergence  exercised  at  a  given  point,  however, 
is  not  the  result  of  accommodation  stimulus,  but  only  about 
two-thirds  of  it.  The  ratio  of  this  accommodative  convergence 
to  accommodation  therefore,  is  about  2  to  1.     The  remaining 


120  CONVERGENCE 

one-third  is  the  result  of  direct  stimulation  by  the  fusion  center. 
Fusion  convergence  is  readily  given  up  under  dissociation  of 
the  images ;  accommodation  convergence  is  not,  so  long  as  the 
demand  for  accommodation  remains. 

RELATIVE  CONVERGENCE. 

As  with  accommodation,  so  with  convergence,  a  certain 
amount  can  be  forced  into  action  after  near  point  has  l)een 
reached  b}'  means  ot  prisms,  base  out,  and  a  certain  amount 
can  be  suppressed  by  means  of  prisms,  base  in,  without  im- 
pairing the  singleness  of  the  image.  This  is  known  as  relative 
convergence. 

NEGATIVE  CONVERGENCE. 

The  office  of  the  external  recti  is  to  undo  the  work  of  the  in- 
ternals, and  to  restore  the  eyes  to  parallelism.  The  power  of 
the  externals,  therefore,  in  this  respect,  is  precisely  equal  to 
that  of  the  internals.  Further  than  this  the  externals  cannot 
normally  be  made  to  act.  By  means  of  prisms,  however,  base 
in,  we  can  force  them  into  action  to  pull  the  eyes  outward  from 
the  median  line,  to  the  extent,  in  normal  eyes,  of  some  8  to  10 
prism  dioptres.  This  action  is  also  spoken  of  by  many  authors 
as  negative  convergence,  but  is  i)erhai)s  better  termed  abduc- 
tion, since  it  has  nothing  whatever  to  tlo  with  convergence. 

There  is  no  basis  of  comparison  between  this  abduction  and 
the  amplitude  of  convergence,  since  the  latter  is  a  normal 
physiological  function,  while  the  former  is  a  forced,  artificial 
feat.  It  can  howe\er  fitly  be  compared  with  the  power  of  ad- 
duction, to  which  it  stands  normal!)  in  the  ratio  of  1  to  3.  In- 
deed, the  principal  \alue  of  these  two  tests — adduction  and  ab- 
duction— is  to  ascertain  the  state  of  this  rclationshiji.  .Abso- 
lute shortages  of  power,  of  c(pial  .imounts,  in  the  two  sets  of 
muscles,  are  not,  as  a  rule,  of  serious  import.  .\  marked  dis- 
turbance of  the  3  to  1  ratio  between  them  usually  indicates 
some  physical  troulije  with  the  musile  that  falls  short  of  its 
(|uota. 

ACCOMMODATIVE   EXOPHORIA. 

W  hen  the  inteni.il  muscles  contract  to  pull  tlu'  i-yi'S  inward 
in  the  act  of  accoinnio(latii  (ii,  a  potential  recoil  is  ilevcloped  in 
the  cxlcrnals  of  precisely   the  same  amount    (a)    to  Imld   the 


CONVERGENCE  121 

eyes  fixed  at  the  point  accommodated  and  converged  for,  and 
(b)  to  restore  the  eyes  to  paralleHsm  as  soon  as  the  tension 
in  the  internals  shall  be  released.  Thus,  with  every  exercise 
of  convergence  an  amount  of  outward  imbalance  is  created, 
equal  to  the  amount  of  convergence  used. 

At  a  near  point,  say  of  25  cm.,  for  example,  about  12  prism 
dioptres  of  convergence  is  exercised,  antagonized  by  a  counter- 
recoil  of  12  prism  dioptres  in  the  externals.  About  two-thirds 
of  this  12  dioptres  of  convergence  is  maintained  by  the  stim- 
ulus of  accommodation ;  the  rest  is  held  in  virtue  of  stimulation 
from  the  fusion  center.  In  other  words,  8  dioptres  is  accommo- 
dation convergence,  4  dioptres  fusion  convergence.  If,  there- 
fore, we  can  dissociate  the  images,  and  destroy  the  fusion 
stimulus,  we  shall  induce  the  patient  to  give  up  4  dioptres  of 
his  convergence. 

This  dissociation  is  very  easily  accomplished.  For  a  test 
chart  we  use  a  black  horizontal  line,  from  the  middle  of  which 
rises  a  perpendicular  index  pointer,  some  half  an  inch  in  height. 
On  either  side  of  the  index  pointer  the  line  is  graded  into  cen- 
timeters. We  place  before  the  right  eye  a  5  or  6  dioptre  prism, 
base  up.  This  immediately  produces  two  images  of  the  line, 
the  false  image  seen  by  the  right  eye  being  about  half  an  inch 
below  the  real  image  seen  by  the  left  eye.  The  images  being 
thus  dissociated,  the  fusion  stimulus,  in  a  few  moments,  ceases 
to  act,  the  eyes  give  up  the  4  dioptres  of  convergence  due  to 
that  stimulus,  and  make  a  turn  outward  of  that  amount.  This 
is  evidenced  by  the  lower  line  on  the  chart  shifting  to  the  op- 
posite side,  so  that  the  index  pointer,  instead  of  standing  imme- 
diately under  the  pointer  of  the  upper  line,  stands  under  the  1 
cm.  mark  on  the  left  hand  side. 

Applying  our  formula. 

Deviation   (in  centimeters) 

Prism  dioptre  =  

Distance   (in  meters) 
we  have  the  following: 

1 
Prism  dioptre  =  —  =  4  p.  d. 
.25 
showing  that  we  have  4  prism  dioptres  of  outward  turn,  mean- 


122  CONVERGENCE 

ing  that  the  eyes  have  surrendered  4  dioptres  of  convergence 
because  of  the  removal  of  the  fusion  stimulus. 

This  amount  of  imbalance,  which  is  thus  manifested  at  the 
near  point  by  dissociation  of  the  images,  is  known  as  accommo- 
dative exophoria.  In  normal  eyes  it  is  a  fairly  constant 
quantity — about  one-third  of  the  calculated  convergence.  In 
ametropia  it  varies,  according  to  the  degree  of  accommodation 
employed ;  and  the  nature  and  extent  of  the  variation  affords 
us  valuable  data  in  our  investigation  of  the  refraction  of  the 
eyes. 

Thus,  a  hyperope  of,  let  us  say,  2  D.,  for  25  cm.  uses  6  D.  of 
accommodation  instead  of  4  D.  At  the  established  ratio,  he 
develops  twice  this  number  of  prism  dioptres  of  accommoda- 
tion convergence,  or  12  prism  dioptres.  That  is  to  say,  all  his 
convergence  at  this  point  is  accommodation  convergence; 
hence  he  will  not  be  likely  to  yield  any  of  it  under  the  dissocia- 
tion test.  A  myope  of  2  I).,  on  the  other  hand,  uses  only  2  D. 
accommodation  at  this  distance,  and  develops  only  4  i)rism 
dioptres  of  accommodation  convergence.  The  rest  of  the  12 
prism  dioptres  employed — 8  p.  d. — must  be  stimulated  by  the 
fusion  center ;  hence,  under  the  dissociation  test  at  near  point 
this  myope  will  jjrobably  give  up  some  8  prism  dioptres  of  his 
convergence,  or,  in  other  words,  will  show  8  prism  dioptres 
of  accommodative  exophoria. 

It  must  not  be  supposed,  however,  that  \  ariations  in  accom- 
modative exophoria  always  work  out  with  this  regularity  in 
their  relation  to  errors  of  refraction,  indeed,  hyperopcs  oc- 
casionally exhibit  undue  exophoria  at  near  point.  Physical 
defects  in  the  muscle  not  infrccpiently  cau.^e  \ariations. 

EXOPHORIA  IN  ACCOMMODATIVE  INSUFFICIENCY. 

Presbyopes  naturally  exhibit  large  degrees  of  accommo- 
dative exophoria,  because  at  near  points  of  33  to  25  cm.  they 
exercise  no  accommodation  at  all.  Their  entire  coiuergence  is 
achieved  in  virtue  of  the  fusion  ciiitir  stimulation. 

liy  the  same  token,  young  i)atients  wlu)  sulTer  from  accom- 
modative insufficiency  are  obliged  to  supplement  their  accom- 
modative stimulus  with  an  extra  powerful  effort  of  the  fusion 
center  in  (jrder  to  maintain  proper  convergence  at  near  point; 


CONVERGENT  STRABISMUS  123 

and  they  yield  correspondingly  larger  amounts  of  exophoria 
under  dissociation  than  do  normal  persons. 

Eberhardt,  indeed,  makes  this  a  standard  test  of  the  suffi- 
ciency of  the  accommodation  for  reading  purposes.  If  the  ac- 
commodative exophoria  is  abnormal,  he  adds  plus  lens  power 
until  it  becomes  normal,  and  prescribes  that  plus  power  for 
reading. 

Convergent  Strabismus.  Strabismus  in  which  the  visual  axes 
are  inclined  toward  each  other.     See  Strabismus. 

Convex.  Curved  with  a  rounded  elevated  surface,  like  the  out- 
side of  a  sphere.  Mathematically  a  surface  is  convex  when, 
if  a  straight  line  be  drawn  from  point  to  point,  the  surface  lies 
between  the  line  and  the  observer. 

Convexo-Concave.  Applied  to  a  lens  w^hich  is  convex  on  one 
side  and  concave  on  the  other,  the  convex  curvature  predomi- 
nating.   See  Meniscus  and  Lens. 

Coordinate.  As  a  verb  the  word  signifies  to  put  into  action 
a  pair  or  a  group  of  muscles,  directed  to  some  single,  purpose- 
ful end.     As  an  adjective,  it  describes  such  a  group-action. 

As  applied  to  the  muscles  of  the  eye,  it  is  usually  employed 
to  indicate  the  use  of  the  muscles  in  homonymous  pairs,  as 
distinct  from  their  conjugate  use. 

Copiopia.     Fatigue  of  the  eye  from  long  or  improper  use. 

Copopsia.    Same  as  Copiopia. 

Coquilles.  Shell-shaped  glasses,  usually  tinted,  for  protection 
of  the  eyes.  Green  states  that  in  order  that  the  lenses  may 
have  a  zero  of  refracting  power  the  concave  surface  should  be 
of  less  curvature  than  the  convex,  the  difiference  between  the 
radii  of  curvature  being  one-third  the  thickness  of  the  glass 
at  its  thickest  point. 

Corectasis.     Dilation  of  the  pupil. 

Corectome.     An  instrument  for  cutting  the  iris. 

Corectopia.     Displaced  pupil. 

Coredialysis.  Detachment  of  the  iris  from  the  ciliary  body  for 
the  purpose  of  making  an  artificial  pupil. 


124  CORELYSIS 

Corelysis.  Detachment  of  the  iris  from  the  cornea  or  the  crys- 
talHne  lens,  to  which  it  has  become  adhered. 

Coremorphosis.     Making  an  artificial  pupil. 

Cornea.  The  transparent  membrane  set  into  the  front  part  of 
the  sclera,  somewhat  like  a  crystal  into  a  watch,  forming  the 
front  window  of  the  interior  chamber.  It  is  the  first  refracting 
medium  through  which  the  light  passes  on  entering  the  eye. 
It  consists  of  five  layers,  from  without  in,  as  follows: 

Epithelium 

Bowman's  Membrane 

Cornea  Proper 

Descemet's  Membrane 

Endothelium 
The  cornea  has  no  blood  vessels,  as  these  would  interfere 
with  its  transparency,  but  is  richly  furnished  with  nerves  in 
Bowman's  meml)rane.  The  epithelial  layer  (sometimes  called 
the  conjunctiva  of  the  cornea)  protects  these  nerves.  The 
cornea  proper  is  composed  of  a  horny,  transparent,  non-sensi- 
tive substance,  which  serves  to  give  shape  and  support.  Des- 
cemet's membrane  and  the  endothelium  protect  the  cornea 
from  within.  The  epithelium  readily  regenerates  itself  after 
injury;  but  Bowman's  membrane,  once  destroyed,  never  re- 
produces itself. 

Optically,  the  cornea  has  a  convex  curvature  anteriorly  and 
a  concave  surface  posteriorly.  Its  index  of  refraction  varies, 
being  greatest  in  the  cornea  projjcr.  The  average  index  is 
1.333.  The  curvature  also  varies  somewhat  in  different  indi- 
viduals, and  is  not  the  same  in  all  meridians.  The  average 
radius  of  the  anterior  surface  cur\aUirc  is  li  mm.  in  the  hor- 
izontal meridian  and  11  mm.  in  the  \ertical.  thus  <)eing 
slightly  more  convex  in  the  latter,  ami  producing  a  slight 
astigmatism,  which,  however,  is  negligible.  The  average 
radius  of  curvature  of  the  i)osteri()r  surface  is  7.5  mm.  The 
cornea  therefore  has  the  elTect  of  a  periscopic  convex  lens  (See 
Lens),  which,  calculating  its  curvatures  and  its  index  of  refrac- 
tion, exerts  a  refracting  power  of  about  43  dioptres.  Imme- 
diately behind  the  cornea  is  the  acjueous  humor,  with  an  index 


CORNEA,  DECENTRATION  OF  125 

of  1.336,  so  that  the  refraction  of  light  passing  from  the  cornea 
to  the  humor  is  practically  nil. 

Cornea,  Decentration  Of.  A  condition  of  the  cornea  in  which  the 
geometric  centre  and  the  optical  centre  do  not  coincide.  It 
is  one  of  the  causes  of  vertical  heterotropia. 

Cornea,  Ectasia  Of.  Bulging  of  the  cornea.  Anterior  staphy- 
loma. 

Corneal  Astigmatism.    See  Astigmatism. 

Corneal  Facets.  Small  flat  areas  of  surface  on  the  cornea, 
usually  due  to  injuries  or  chronic  ulcers. 

Corneal  Reflex.    Winking  of  the  eye  when  the  cornea  is  touched. 

Corpora  Quadrigemina.  See  above.  These  bodies  are  four  in 
number,  as  their  name  implies. 

Corporata  Geniculata.  Two  important  ganglia  in  the  mid-brain 
which,  together  with  the  quadrigeminal  bodies  and  the  optic 
thalamus,  constitute  the  first  relay  station  in  the  transmission 
of  the  visual  impulse  from  the  retinae  along  the  optic  tract. 

Corpuscular  Light  Theory.  The  theory  held  by  Newton  that 
light  consisted  of  minute  corpuscles  of  matter  which  bom- 
barded the  retina.    See  Light. 

Corpus  Vitreum.    The  vitreous  body. 

Correction.  In  optics  this  term  is  applied  to  the  lens  power,  or 
combination  of  lens  power,  which,  placed  before  the  ametropic 
eye,  renders  it  emmetropic. 

Corresponding  Points  of  the  Retinae.  Coincident  points  in  the 
two  retinae,  i.  e.  at  the  same  relative  distances  and  positions 
from  the  visual  centres,  upon  which  light  waves  from  identical 
points  of  an  object  fall,  producing  single  vision.  (See  Binocu- 
lar Vision).  They  are  also  called  identical  points  of  the 
retinae. 

Cortical.  The  cortex  of  an  organ  is  the  part  that  lies  nearest  to 
the   outer   border ;   the   opposite   of   the    nucleus.      Anything, 


126  CORUSCATION 

therefore,  is  cortical  which  pertains  to  the  outer  border  of  an 
organ,  as  a  cortical  cataract,  involving  the  circumferential  por- 
tion of  the  lens. 

Coruscation.    A  sensation  of  a  Mash  of  light  before  the  eyes. 

Couching.  Operation  by  which  the  lens  is  displaced  out  of  the 
line  of  vision  by  use  of  the  couching  needle. 

Cover  Test.  A  test  for  muscular  imbalance,  made  by  covering 
the  sound  eye  suddenly  while  both  eyes  are  fixing  a  point. 
If  there  is  no  imbalance,  the  uncovered  eye  will  remain  sta- 
tionary; if  there  is  imbalance,  it  will  make  a  slight  turn,  so  as 
to  get  a  new  fixation  status. 

Crisis,  Ocular.  Se\"ere  pain  in  the  e}es  and  brow  sometimes 
occurring  in  locomotor  ataxia. 

Critical  Angle.     Sec  Angle. 

Crossed  Cylinders.  Two  cylindrical  lenses  placed  in  ajjposition 
with  their  axes  at  right  angles  to  each  other.  Two  e(iual  and 
similar  cylinders  thus  placed  would,  of  course,  make  the  opti- 
cal equivalent  of  a  spherical  lens.  The  term,  as  generally  used, 
therefore,  usually  implies  cylinders  of  unecpial  power  or  unlike 
curvature. 

The  use  of  cross-cylinders,  as  originated  by  Jackson,  for  the 
testing  of  presbyopia,  and  later  by  Lockwood,  for  finding  the 
comfortable  near-point,  will  bt-  found  described  in  detail  under 
the  heading  of  Accommodation. 

Crossed  Diplopia.  I  )iplopia  in  which  the  image  in  the  ile\  iating 
eye  is  projected  to  the  op])osite  \  isual  field.     .See  Diplopia. 

Cross-Eyed.     lla\ing  a  strabismus. 

Crossed  Eyes.     .\  i-omnion  Icrni  for  strabismus. 

Cross-Hair.  .\  thin  tlinad  or  wire  stretched  .icross  the  f(»cal 
plane  of  ;'.n  ojilical  iiistrununt  for  purposes  of  localization. 
Two  such  strands  are  often  stntched  at  right  anglis  to  eacli 
other,  in  (jrder  to  localize  a  point  where  tlu-y  intersect. 


CROSSED  LENS  127 

Crossed  Lens.  A  lens  which  shows  the  minimum  of  aberration 
for  neutral  waves.  Such  a  lens  is  a  bi-convex  or  a  bi-concave 
whose  front  surface  has  a  curvature  six  times  as  great  as  that 
of  its  back  surface.  The  plano-convex  is  almost  as  good.  The 
crossed  lens,  when  made  of  flint  glass  of  1.6  index,  is  a  plano- 
convex. 

Cross,  Maddox.  A  cross  device  designed  by  Ernest  Maddox, 
the  vertical  and  horizontal  arms  being  graduated  in  centi- 
meters, for  measuring  vertical  and  lateral  heterophoria  and 
squint. 

Cross-Section.  A  section  made  at  right  angles  to  the  principal 
axis. 

Cryptophthalmia.  A  congenital  malformation,  consisting  of 
adhesion  of  the  lids  to  the  eyeballs,  or  of  the  lids  to  each  other. 

Crystalline  Lens.     (See  Lens,  Crystalline). 

Cul-De-Sac.  A  blind  pouch.  There  are  many  cul-de^sacs  in 
human  anatomy.  In  ophthalmology  the  term  is  applied  to 
the  sac  formed  by  the  bulbar  and  the  palpebral  conjunctiva. 

Cupped  Disc.  A  depression  or  excavation  of  the  optic  disc, 
apparent  with  the  ophthalmoscope  by  reason  of  the  fact  that 
the  edge  and  centre  of  the  disc  cannot  be  focussed  simultane- 
ously.    There  are  three  varieties : 

Physiologic,  due  to  the  separation  of  the  optic  nerve  fibres 
behind  the  retina  instead  of  in  the  same  plane.  It  is  never  com- 
plete, and  the  lamina  cribrosa  is  in  normal  position.  The  rest 
of  the  eye-ground  is  normal. 

Atrophic,  caused  by  disappearance  of  the  nerve  fibres.  Ex- 
cavation is  total  but  shallow,  the  lamina  c*ibrosa  is  in  place 
and  the  area  is  white.     (See  Optic  Atrophy). 

Glaucomatous,  originating  in  the  recession  of  the  lamina 
cribrosa,  due  to  intraocular  tension.  The  excavation  is  total 
and  deep,  as  shown  by  the  bending  of  the  vessels  as  they 
emerge  from  the  cup.  In  early  stages  the  nerve  head  is  still 
pink,  but  later  becomes  pale.  There  are  other  signs  of  glau- 
coma {q.  v.). 

Curtometer.     An  instrument  for  measuring  curved  surfaces. 


128  CURVATURE 

Curvature.  The  gradual  bending  of  a  surface  without  making 
an  angle.  It  is  the  uniform  curvature  of  a  refracting  surface 
which  enables  it  to  bring  light  to  a  focus  at  one  point.  Other 
things  being  equal,  the  greater  the  degree  of  curvature,  the 
greater  the  refracting  power.  Degree  of  curvature  is  expressed 
in  two  ways,  (1)  in  terms  of  the  meter  curve,  i.  e.  the  degree 
of  curvature  of  a  sphere  whose  radius  is  1  meter,  and  (2)  in 
terms  of  the  radius  of  the  curve. 

Cyanopsia.    A  condition  in  which  everything  is  seen  blue. 

Cyclitis.  Inflammation  of  the  ciliary  body  of  the  eye.  A  very 
serious  condition,  which  almost  always  involves  the  entire 
internal  structure  of  the  eye,  giving  great  pain,  dimness  of 
vision,  and  cloudy  media.  Light  is  painful  because  it  causes 
the  ciliary  muscle  to  contract.  As  the  chorioid,  the  iris,  and 
ciliary  are  all  parts  of  the  same  tissue-system,  inflammation 
of  one  is  frequently  accompanied  b>'  inflammation  of  the  other 
two. 

Cyclodialysis.  An  operation  for  detachment  of  the  ciliary  body 
at  its  periphery. 

Cyclophorometer.  An  instrument  for  measuring  cyclophoria. 
The  most  generally  used  consists  of  two  metal  disks,  con- 
nected by  a  rigid  bar,  with  a  Maddox  rod  in  the  centre  of  each, 
which  can  he  rotated  to  any  desired  angle. 

Cyclopia.     A  single  eye  in  the  middle  of  the  forehead. 

Cyclophoria.  Imbalance  of  the  extrinsic  muscles  of  the  eye  in 
which  the  oblicjuc  muscles  play  a  dominant  role,  causing  the 
eye  to  (le\iate.  under  test,  ol)li(|uely.     (See  Heterophoria). 

Cycloplegia.  I'aralysis  of  the  ciliary  muscle,  so  thai  the  pupil 
is  dihited  and  acconimodatit)n  is  suspended.  This  condition  is 
frecjuently  brouj^ht  ab(jut  artificially,  by  means  of  a  drug, 
eitluT  to  jirocure  rest  for  tin-  muscle  in  intlaniniatiou,  or  to 
enable  the  refraction  of  the  eye  to  be  measined  without  any 
accommodation  in  force.  The  drug  most  commonly  used  in 
the  first  class  of  cases  is  atroi)inf  ;  in  the  sicond  class,  Immatro- 
l)ine  ;  the  former  drug  being  profound  and  lasting  in  elTect.  the 


CYCLOPLEGIC  129 

latter  light  and  transient.     Cocaine  is  also  a  cycloplegic,  but  is 
not  used  for  that  purpose. 

Cycloplegic.  A  drug  which  paralyzes  the  ciliary  muscle.  (See 
above). 

Cylinder.  This  geometric  figure  is  difficult  to  define  mathema- 
tically. It  is  a  long,  round  solid  body,  terminating  at  each 
end  in  a  flat,  circular  surface,  these  two  surfaces  being  parallel 
to  each  other.  In  right  cylinders  the  straight  line  joining  the 
centres  of  the  two  bases  is  perpendicular  to  the  bases,  in 
oblique  cylinders,  it  makes  an  acute  angle  with  them. 

Optically,  the  importance  of  a  cylinder  is  that  its  surface 
presents  a  uniform  crescendo  of  refracting  power,  from  the 
minimum,  along  its  cylinder  axis  (see  Axis),  to  the  maximum, 
at  right  angles  to  its  cylinder  axis.  The  minimum  power  is 
always  zero,  since  it  has  no  curvature  parallel  with  its  axis ; 
the  maximum  power  depends  upon  the  radius  of  the  circular 
aspect.  Intermediate  refracting  power  is  in  direct  proportion 
to  the  angular  distance  of  the  meridian  in  question  from  the 
axis.  Thus,  at  30  deg.  from  the  axis,  which  is  one-third  of 
ninety  deg.,  the  cylinder  has  one-third  of  its  maximum  power; 
at  45  deg.  one-half,  etc. 

Another  peculiar  property  of  a  cylinder  is  that,  so  far  as 
refraction  is  concerned,  it  represents  the  split  half  of  a  sphere. 
Thus,  if  two  cylinders,  of  equal  radius,  be  placed  in  apposition 
with  their  axes  at  right  angles  to  each  other,  every  meridian 
is  so  reinforced  by  the  combination  as  to  give  to  every  meri- 
dian the  maximum  refracting  power  of  either  cylinder,  thus 
giving  to  the  combination  the  refracting  value  of  a  sphere. 
This  gives  a  cylindrical  lens,  (made  of  a  segment  of  a  cylinder), 
the  power  of  correcting  regular  astigmatism.  (See  Astigma- 
tism and  Lens). 

Dacryadenalgia.    Pain  in  the  lacrymal  gland. 

Dacryagogatresia.    Closure  of  a  lacrymal  duct. 

Dacryagogue.     A  medicine  which  promotes  the  flow  of  tears. 

Dacryma.     A  tear. 


130  DACRYOADENITIS 

Dacryoadenitis.     Inflammation  of  the  lacrymal  gland. 

Dacryocele.    A  cyst  of  the  lacrymal  sac. 

Dacryocyst.    See  above. 

Dacryocystalgia.     Pain  in  the  lacrymal  sac. 

Dacryoma.     A  tumor  of  the  lacrymal  organs. 

Dacryon.     The  lacrymal  point. 

Dacryops.     Distention  of  the  lacrymal  sac. 

Dacryopyorrhea.    A  flow  of  pus  from  the  lacrymal  sac. 

Dacryopyosis.     A  mixture  of  tears  and  pus. 

Dacrycystitis.  Inflammation  of  the  tear  duct,  which  may  he- 
come  infected  either  from  the  conjunctiva  or  from  the  nose. 
There  is  usually  stoppage  of  the  duct,  so  that  the  tears  over- 
flow the  eye,  the  serum  or  pus  exudes  from  the  duct  itself. 

Daltonism.  Another  name  for  color  blindness,  so  called  because 
Dalton  first  demonstrated  it. 

Darkness  Acuity.  The  acuity  of  vision  in  comparati\e  darkness 
(jr  low  degrees  of  light. 

Day-Blindness.  A  condition  in  which  vision  is  better  at  night 
than  in  tlu  day-time.  It  ma\-  be  due  to  a  retinal  trouble,  in 
which  the  briglit  light  of  day  impairs  fimction,  ov  to  a  central 
opacity  of  the  lens,  so  that  when  the  pupil  dilates  luuler  the 
influence  of  dusk  an  aperture  is  made  ari)und  the  ojiacity  tor 
the  entrance  of  light. 

Decentration.  A  lens  is  said  to  be  centered  wiien  its  ojitical 
centre  coincides  with  the  visual  axis  of  tiie  eye;  and  as  llie 
visual  axis  of  the  eye  in  a  stati'  of  rest  is  supposed  to  pass 
through  tlie  geometrical  centre  of  the  lens,  a  lens  is  regardeil 
as  being  centered  when  its  giduutrical  and  its  optical  centres 
coincide.  Wiicn  this  is  not  the  ca^e.  i.  v.  when  the  optical 
centre  is  to  one  side  or  other  of  the  geonu-trical  centre,  the 
lens  is  said    to   lu'   ileceutered.      When    tlu-  optical    centre   is   to 


DECENTRATION  131 

the  inner  side  of  the  geometrical,  it  is  said  to  be  "decentered 
in'';  when  the  reverse,  it  is  "decentered  out." 

PRISM  EFFECT  OF  DECENTERING. 

It  can  be  readily  seen  that  the  effect  of  a  spherical  lens 
which  has  been  decentered  is  that  of  a  prism  before  the  eye, 
because  at  any  point  in  a  spherical  lens  other  than  its  optical 
centre  the  plans  of  the  two  surfaces  are  not  parallel,  but  are 
inclined  to  each  other  at  an  angle  depending  upon  the  degree 
of  curvature.  When,  therefore,  the  visual  axis  passes  through 
a  lens  at  any  other  point  than  the  optical  centre  the  effect  is 
the  same  as  looking  through  a  prism.  Whether  the  prism  be 
base  in  or  out,  up  or  down,  depends  upon  whether  the  sphere  is 
a  convex  or  a  concave  one,  and  upon  the  direction  of  displace- 
ment. 

Since  a  convex  lens  is  thickest  at  its  optical  centre,  the  base 
of  the  prism  produced  by  its  decentering  always  is  toward  the 
optical  centre  of  the  lens;  hence  when  a  convex  lens  is  decen- 
tered, the  base  of  the  prism  is  in  the  same  direction  as  the 
decentering.  A  concave  lens  being  thinnest  at  its  optical 
centre,  the  prism  produced  by  its  decentering  has  always  its 
base  away  from  the  optical  centre ;  hence  when  a  concave  lens 
is  decentered  the  base  of  the  prism  is  in  the  opposite  direction 
of  the  decentering. 

As  a  decentered  spherical  lens  has  the  double  effect  of  a 
spherical  lens  of  the  strength  indicated,  plus  a  prism  of  the 
character  and  strength  corresponding  to  the  degree  of  decen- 
tration,  and  as  it  is  often  desirable  to  combine  a  prism  with  a 
sphere,  lenses  are  often  prescribed  to  be  decentered  so  as  to 
give  the  required  prism  effect. 

AMOUNT  OF  PRISM  EFFECT  PRODUCED. 
The  stronger  the  lens,  of  course,  the  smaller  the  degree  of 
decentration  needed  to  produce  a  given  prism  effect.  Approx- 
imately 1  prism  dioptre  is  produced  by  decentering  a  1  D. 
lens  1  cm.  Hence,  in  determining  the  amount  of  decentration 
necessary  to  produce  a  given  number  of  prism  dioptres  with 
a  lens  of  a  given  strength,  we  simply  divide  the  number  of 
prism  dioptres  desired  by  the  dioptrism  of  the  lens,  and  the 
quotient  is  the  number  or  the  fraction  of  the  centimeters  of 


132  DECENTRATION 

decentration  necessary.  For  instance,  if  it  is  desired  to  pro- 
duce 2  prism  dioptres  with  a  4  D.  lens,  we  divide  4  into  2, 
which  gives  0.5  ;  the  necessary  decentration  is  0.5  cm.,  or  5  mm. 
Following  is  a  table  of  the  amount  of  prism  power  obtained 
by  decentering  spherical  lenses  for  each  dioptre  of  lens  power. 
The  same  table  holds  good  for  decentering  cylindrical  lenses 
provided  the  decentering  be  done  in  a  direction  at  right  angles 
to  the  axis  of  the  cylinder;  if  the  cylinder  be  decentered  in  any 
other  direction  its  prism  efifect  must  be  calculated  according 
to  the  meridian  along  which  the  decentration  takes  •  place. 
Decentration  along  the  axis,  of  course,  produces  no  prism 
effect,  as  there  is  no  lens  power  in  that  direction. 

Prism  Dioptres  Produced  by  Decentration  Per  Millimeter. 


Diopt 

Im/ni  2m/m 

3m/m 

4ni/m  5m/in 

6ni/m 

7m/m 

8m/ni  9 

m/m  lOm/m 

0.25 

.025 

.050 

.075 

1.00 

.125 

.150 

.175 

.200 

.225 

TSO 

0.50 

.05 

.10 

.15 

.20 

.25 

.30 

.35 

.40 

.45 

.50 

0.75 

.075 

.15 

.225 

.30 

.375 

.45 

.525 

.60 

.675 

.75 

1.00 

.10 

.20 

.30 

.40 

.50 

.60 

.70 

.80 

.90 

1.00 

1.25 

.125 

.25 

.375 

.50 

.625 

.75 

.875 

1.00 

1.125 

1.25 

1.50 

.15 

.30 

.45 

.60 

.75 

.90 

1.05 

1.20 

1.35 

1.50 

1.75 

.175 

.35 

.525 

.70 

.875 

1.05 

1.225 

1.40 

1.575 

1.75 

2.00 

.20 

.40 

.60 

.80 

1.00 

1.20 

1.40 

1.60 

1.80 

2.00 

2.25 

.225 

.45 

.675 

.90 

1.125 

1.35 

1.575 

1.80 

2.025 

2.25 

2.50 

.25 

.50 

.75 

1.00 

1.25 

1.50 

1.75 

2.00 

2.25 

2.50 

2.75 

.275 

.55 

.825 

1.10 

1.375 

1.65 

1.925 

2.20 

2.475 

2.75 

3.00 

.30 

.60 

.90 

1.20 

1.50 

1.80 

2.10 

2.40 

2.70 

3.00 

3.25 

.325 

.65 

.975 

1.30 

1.625 

1.95 

2.275 

2.60 

2.925 

3.25 

3.50 

.350 

.70 

1.05 

1.40 

1.75 

2.10 

2.45 

2.80 

3.15 

3.50 

3.75 

.375 

.75 

1.125 

1.50 

1.875 

2.25 

2.625 

3.00 

3.375 

3.75 

4.00 

.40 

.80 

1.20 

1.60 

2.00 

2.40 

2.80 

3.20 

3.60 

4.00 

4.25 

.425 

.85 

1.275 

1.70 

2.125 

2.55 

2.975 

3.40 

3.825 

4.25 

4.50 

.45 

.90 

1.35 

1.80 

2.25 

2.70 

3.15 

3.60 

4.05 

4.50 

4.75 

.475 

.95 

1.425 

1.90 

2.375 

2.85 

3..^25 

3.80 

4.275 

4.75 

5.00 

.50 

1.00 

1..S0 

2.00 

2.50 

3.00 

3.50 

4.00 

4.50 

5.(X) 

5.25 

.525 

1.05 

1.575 

2.10 

2.625 

3.15 

3.075 

4.20 

4.725 

5.25 

5.50 

.55 

1.10 

1.65 

2.20 

2.75 

3.30 

3.85 

4.40 

4.95 

5.50 

5.75 

.575 

1.15 

1.725 

2.30 

2.875 

3.45 

4.025 

4.00 

5.175 

5.75 

6.00 

.60 

1.20 

1.80 

2.40 

3.00 

3.60 

4.3) 

4.80 

5.40 

6.00 

When  it  is  desired  to  decentre  a  lens  both  horizontally  and 
\  crtically  at  the  same  time,  the  effect  can  be  obtained  by  per- 
forming a  resultant  decentration  in  ;in  obliciue  direction  be- 
tween the  horizontal  and  the  vi'iticil.  l"\)r  such  a  resultant 
decentration  the  amount  of  prism  power  ri"(|uirt.'d  is  found  by 
adding  togetluT  the  S(iuares  of  the  horizontal  and  vertical 
j)risms  and  extracting  the  s(|uare  root  of  the  sum,  and  the  posi- 
tion of  the  base  of  the  resultant  prism  is  dctcrmincil  according 
U)  the  following  formula: 


DECIMETER  133 

Horizontal  prism  X  90 


degrees  from  the  vertical. 


Total  prism 

(In  the  foregoing  formula  the  expression  "total  prism"  re- 
fers to  the  sum  of  the  prism  power  in  the  horizontal  meridian 
and  that  in  the  vertical  meridian.) 
Decimeter.    One-tenth  of  a  meter.    About  4  inches. 

Declination.  This  term  is  applied  to  the  deviation  of  the  vertical 
axis  of  the  eyeball  when  the  latter  turns  on  its  antero-posterior 
axis.  Such  action  is,  of  course,  due  to  the  action  of  the  oblique 
muscles.  When  the  upper  end  of  the  vertical  axis  inclines 
toward  the  nose,  it  is  called  positive  declination ;  when  toward 
the  temples,  negative  declination. 

Decomposition.  As  applied  to  light,  the  word  signifies  the 
breaking  up  of  the  white  pencil  of  light  into  its  constituent 
color  waves,  either  by  means  of  a  prism  or  by  diffraction.  (See 
Prism  and  Diffraction). 

Decussation.  A  crossing  or  intersection  of  lines  or  paths.  Rays 
of  light  decussate  at  the  point  of  reversal ;  but  as  rays  are  only 
geometrical  lines,  used  to  illustrate  the  direction  of  light- 
waves, we  cannot  properly  say  that  light  decussates.  The 
term,  in  fact,  does  not  technically  belong  to  optics,  but  to 
physiology,  where  it  is  applied  to  the  crossing  or  intersection 
of  nerve  fibres.  There  is  a  partial  decussation  of  the  fibres  of 
the  optic  tract  at  the  chiasm,  q.  v. 

Density,  Optical.  The  property  possessed  by  bodies  or  substan- 
ces of  retarding  the  velocity  of  light  waves. 

Dennett's  Prism  Nomenclature.  The  system  of  measuring 
prism  deviation  in  angles  consisting  of  a  hundredth  of  a 
radian.    See  Prism. 

Dental  Amblyopia.    Amblyopia  due  to  diseases  of  the  teeth. 

Deorsumvergence.    Turning  of  the  eyes  downward. 

Depilation.    Removal  of  hair. 

Depth.  In  optics  this  term  denotes  the  quality  of  solidity,  as 
perceived  by  the  vision.    See  Binocular  Vision. 


134  DEPLUMATION 

Deplumation.     Loss  of  eyelashes. 

Deprimens  Oculi.    Another  name  for  the  inferior  rectus  macula. 

Descemetitis.     Inflammation  of  the  Descemet's  membrane. 

Descemet's  Membrane.  The  fourth  layer  of  the  cornea,  which 
lies  immediately  behind  the  cornea  proper.  Also  called  the 
internal  limiting  membrane.     (See  Cornea). 

Deviation.  Literally,  turning  aside.  Used  in  optics  to  denote 
(1)  the  bending  of  the  path  of  a  light  wave  from  its  original 
course  by  the  action  of  a  prism,  and  (2)  the  turning  of  the 
visual  axis  in  or  out  from  the  median  line  in  convergence. 
(See  Prism  and  Convergence).  In  the  former  case,  deviation 
(in  centimeters)  is  equal  to  the  distance  from  the  prism  (in 
meters)  at  which  the  calculation  is  made,  multiplied  by  the 
dioptric  power  of  the  prism.  Thus,  a  2  dioptre  prism  gives  a 
tieviation  of  6  centimeters  at  3  meters  distance  from  the  prism. 
Or  deviation  may  be  measured  in  terms  of  the  angle  of  devia- 
tion, or  by  the  sine  of  that  angle. 

Primary  and  secondary  deviation  are  terms  used  in  connec- 
tion with  strabismus.  Primary  deviation  is  the  deviation  made 
by  the  strabismic  eye  when  the  sound  one  is  fixing.  Secondary 
deviation  is  the  deviation  made  by  the  sound  e}e  when  the  bad 
one  is  fixing. 

Conjugate  deviation  is  the  turning  of  both  eyes  at  once  in 
the  same  direction,  as  when  we  turn  them  to  look  to  right  or 
left.  In  certain  diseases  of  the  brain  there  is  constant,  invol- 
untary conjugate  de\iation. 

Deviometer.  An  instrument  for  nu-asuring  the  deviation  of  the 
eye  in  strabismus.     See  Strabismus. 

Dextrocular.  Kiglil-e\  ed,  i.  f.  iia\  iiig  the  right  eye  the  iloininant 
eye. 

Dextroduction.     MoNcnicnt  towards  the  right  of  the  visual  axis. 

Pextrophoria.  A  condition  of  conjugate  inilialaiicf,  in  which  the 
two  eyes  tend  to  turn  to  the  right. 


DIACAUSTICS  135 

Diacaustics.     See  Caustic. 

Diameter.  A  straight  line  joining  two  opposite  points  on  the 
circumference  of  a  sphere  or  circle,  passing  through  the  centre. 

Diaphanometer.  An  instrument  for  estimating  the  amount  of 
solids  in  a  fluid  by  measuring  its  transparency. 

Diaphonoscope.  Diaphonoscopy.  An  instrument  and  method  of 
examinatihg  tissues  by  transillumination. 

Diaphragm.  A  muscular  or  membranous  curtain,  dividing  two 
open  spaces  or  chambers.  In  the  eye,  the  iris  fulfills  this 
description. 

Diapyesis.     Suppuration.  ' 

Dichromate.  A  color-blind  person  who  can  distinguish  but  two 
colors,  usually  complementary  colors. 

Dichromatic.     Seeing  two  colors  at  one  time. 

Dichromatism.  If  white  light  is  allowed  to  fall  upon  certain 
colored  solutions,  the  transmitted  light  is  of  one  color  when 
the  thickness  of  the  solution  is  small,  and  quite  another  color 
when  the  thickness  is  great.  Thus,  if  a  beam  of  white  light  be 
projected  through  a  solution  of  chlorophyll,  if  the  thickness  of 
the  solution  be  not  great  it  is  seen  on  the  other  side  as  green ; 
but  if  the  thickness  of  the  solution  be  considerable,  then  it 
appears  on  the  other  side  as  quite  a  deep  red.  The  explana- 
tion is  as  follows :  The  solution  is  moderately  transparent 
for  a  large  number  of  rays  in  the  spectral  neighborhood  of 
green,  and  for  only  a  few  red.  The  small  amount  of  red  is  at 
first  overpowered  by  the  large  amount  of  green,  but,  having 
a  smaller  coefficient  of  absorption,  it  finally  becomes  predom- 
inant if  it  travels  through  a  sufficient  thickness.  This  phe- 
nomenon is  known  as  dichromatism. 

Diffraction.  A  term  applied  to  the  modification  which  rays  of 
light  undergo  when  they  pass  over  the  edge  of  an  opaque 
body.  Thus,  when  a  beam  of  light  is  admitted  into  a  dark 
chamber  through  a  narrow  slit,  and  falls  upon  a  screen,  there 
appears  a  line  of  white  light  bordered  by  a  fringe  of  alternate 


136  DIFFUSION 

colored  light  and  dark  ;  this  fringe  is  prochiced  by  the  decom- 
position of  the  light  by  the  edge  of  the  substance  through 
which  it  passes,  and  is  known  as  diffraction  bands. 

The  original  light  waves  are  known  as  primary,  and  the 
waves  resulting  from  the  diffraction,  secondary  waves.  It 
appears  that  the  secondary  waves  possess  similar  properties 
to  the  extraordinarily  refracted  waves  of  double-refracting 
crystals,  i.  e.  they  are  polarized.  By  means  of  millions  of  tiny 
perforated  lines  in  an  opaque  diaphragm,  placed  exceedingly 
close  together,  light  may  be  thus  thoroughly  decomposed. 
Such  a  device  is  known  as  a  "grating."'  and  practically  the 
entire  light  that  passes  through  the  grating  is  polarized.  (See 
Polarization). 

Diffraction  of  light  gives  rise  to  many  of  the  most  abstruse 
and  complicated  problems  in  optics. 

Diffusion.  A  gradual  and  thorough  disjiersion  of  light  among 
the  particles  of  some  absorbing  or  transparent  medium.  The 
light  of  the  sun  is  thus  diffused,  normally,  thrDugh  the  air. 
By  this  means  a  \ery  uniform  illumination  is  obtained,  and 
modern  artilicial  lighting  aims  to  imitate  it.  Diffused  light 
must  be  ro-i)olarized  and  re-f(jcalized  before  it  can  produce  an 
image. 

Diffusion  Circle.  Circles  of  light  presented  by  spherical  wa\es  of 
light  at  any  place  in  their  course  other  than  their  focal  jjoint. 
These  circles  of  diffusion  fall  on  the  retina,  insti-ad  of  focal 
points,  in  ametropia. 

(leo.  A.  Rogers  ])oints  out  tliat  the  diffusion  circles  which 
fall  on  the  retina  in  hyperopia  and  tbost'  in  myopia  an-  not 
alike,  since  the  former  ct)nsist  of  diminishing  (minus)  waves, 
whose  jieripheries  strike  the  retina  first,  while  the  former  con- 
sist of  expanding  (jjIus)  waves,  whose  apices  strike  first;  this, 
however,  can  hardly  ha\e  any  practical  significance. 

Dilatation.  l"-xpansion.  Applied  to  a  hollow  \csscl  or  organ. 
W  lull  ;in  aperture  is  in  (|uestion.  \\  c  use  the  term  Dilation,  as 
of  till'  pupil. 

Dilator.  Iliis  word  has  two  a|ipliialions  in  physiology.  I'irst. 
it  is  a|)plicd  to  a  niuscle.  or  set  of  iuumIi'S.  which  brim;  about 


DILATOR  IRIDIS  137 

a  dilatation  of  the  organ  or  aperture  which  they  supply ;  thus, 
the  radiating  muscles  of  the  iris  are  called  irido-dilators.  Sec- 
ond, it  is  applied  to  a  drug  or  other  agent  which  causes  dilata- 
tion, e.  g.  atropine,  cocaine,  etc. 

Dilator  Iridis.    The  radiating  muscle  which  dilates  the  pupil. 

Dionin.  A  derivative  of  opium  which,  in  a  1  per  cent  to  5  per 
cent  solution  is  used  in  the  eye  to  promote  glandular  activity. 
It  is  useful  in  incipient  cataract,  iritis,  cyclitis,  etc. 

Dioptometer.  Dioptrometer.  An  instrument  for  measuring  re- 
fraction. 

Dioptometry.  Dioptrometry.  Measurement  of  the  refraction  of 
the  eye. 

Dioptre.  The  unit  of  focalizing  power.  One  dioptre  is  the 
power  to  focalize  a  neutral  wave  of  light  at  a  distance  of  1 
meter.     (See  Lens). 

Dioptrics.    The  science  of  light  refraction. 

Dioptroscopy.  Measurement  of  refraction  by  means  of  the  oph- 
thalmoscope. 

Diplocoria.     A  double  pupil  in  the  eye. 

Diplomometer.     An  instrument  for  measuring  diplopia. 

Diplopia.  Double  vision,  due  to  the  fact  that  the  central  rays  of 
light  from  the  object  do  not  fall  coincidently  on  the  yellow 
spots  of  the  two  retinae.  In  the  outer,  peripheral  areas  of  the 
visual  field  there  is  always  double  vision,  as  the  rays  from 
these  areas  never  fall  on  identical  retinal  points.  This,  how- 
ever, does  not  concern  the  individual,  provided  the  central 
parts  of  the  image  coincide.  Only  when  the  central  images 
are  out  of  coincidence  does  he  complain  of  diplopia. 

Diplopia  is  said  to  be  homonymous  when  the  two  images 
appear  to  be  on  the  same  sides,  respectively,  as  the  eyes  which 
see  them ;  heteronymous  when  they  appear  to  be  on  opposite 
sides ;  the  latter  is  also  called  crossed  diplopia.  The  image 
seen  by  the  fixing  eye  is  termed  the  true  image ;  that  which  is 


138  DIPLOPIA,  PHYSIOLOGIC 

seen  by  the  deviating  eye,  the  false  image.  It  is  to  be  remem- 
bered that  the  apparent  displacement  of  the  false  image  is 
always  in  the  opposite  direction  to  that  in  which  the  eye  dev- 
iates. \"ertical  displacement  occurs  when  the  visual  axes  are 
not  parallel,  or  when  the  eyes  stand  at  different  levels. 

When  one  or  both  eyes  are  twisted  on  the  saggital  a.xis,  the 
upper  or  lower  extremities  of  an  object  may  approximate,  so 
that  the  image  takes  the  form  of  an  upright  or  inverted  V. 

Since  diplopia  is  the  subjective  manifestation  of  heterotro- 
pia,  or  strabismus,  its  diagnosis,  measurement,  and  general 
management  are,  of  course,  those  of  strabismus,  q.  v.  It  may 
be  either  paralytic  or  functional.  Below  will  be  found  a  table 
showing  the  displacement  of  the  image  and  the  behavior  of 
the  diplopia  in  the  disablement  of  the  various  ocular  muscles. 
The  tal)le  is  meant  to  apply  to  paralytic  diplopia,  but  it  holds 
good  in  general  for  functional  diplopia,  except  that  in  the  func- 
tional cases  there  is  no  increase  of  diplopia  upon  making  con- 
jugate movements : 

Key  to  table.     DH     Homonymous  diplopia 
BX     Crossed  diplopia 
DR     Right  vertical  diplopia 
DL     Left  vertical  diplopia 
Er      Eyes  right 
El       Eyes  left 
Eu      Eyes  up 

Ed      Eyes  down         inc.  denotes  increased 
R.  external  rectus         DH  inc.  in  Er 
L.  internal  rectus         DX  inc.  in  Er 
R.  internal  rectus         DX  inc.  in  El 
L.  external  rectus        DH  inc.  in  El 
R.  superior  rectus        DL  inc.  in  Eu  and  r. 
L.  inferior  oblique        DR  inc  in  Eu  and  r. 
R.  inferior  oblicjue        DL  inc.  in   Eu  and   1. 
L.  superior  rectus        DR  inc.  in   lui  and  1. 
R.  inferior  rectus  DR  inc.  in   Ed  and  r. 

L.  superior  oblitiue  DL  inc.  in  Kd  and  r. 
R.  superior  oblique  DR  inc.  in  Ed  and  1. 
L.  inferior  rectus  DL  inc.  in   I'M  and  1. 

For  further  discussion  of  this  subject  the  reader  is  referred 
to  the  section  on  Strabismus. 

Diplopia,  Physiologic.     Double  vision  which  normallv  occurs  of 


DIPLOSCOPE  139 

those  portions  of  the  visual  field  which  are  outside  the  horop- 
ter.    See  Binocular  Vision. 

Diploscope.  An  instrument  devised  by  Remy  for  the  determina- 
tion of  binocular  vision  and  detecting  malingering,  based  upon 
the  bar-reading  principle  of  Javal.  It  consists,  essentially,  of 
two  tubes,  with  a  broad  bar  across  them,  through  which  the 
patient  reads  a  distance  type  chart. 

Direct  Image.  Image  of  the  fundus  seen  by  the  direct  method 
of  ophthalmoscopy. 

Direct  Method.     See  Ophthalmoscopy. 

Direct  Vision.  Vision  in  which  the  focussed  light  waves  fall 
upon  the  macula. 

Discission.  Needling  the  capsule  of  the  crystalline  lens  in  soft 
cataract. 

Discoria.    Same  as  Diplocoria. 

Disc,  Optic.  A  white,  circular  area  seen  with  the  ophthalmo- 
scope on  the  retina,  about  1.5  mm.  in  diameter,  a  little  to  the 
nasal  side.  It  represents  the  place  where  the  optic  nerve  enters 
the  eye,  and  is  itself  impervious  to  light  stimulation;  whence 
it  is  also  known  as  the  blind  spot.  It  is  slightly  raised  from 
the  level  of  the  retina. 

The  disc  is  one  of  the  chief  landmarks  and  points  of  interest 
in  an  ophthalmoscopic  examination  of  the  eye.  In  order  to 
view  it,  the  light  must  be  thrown  into  the  eye  slightly  to  the 
temporal  side  of  the  cornea.  In  health  it  appears  a  light  pink, 
tinged  with  yellow,  of  a  lustrous  texture.  Around  its  circular 
edge  can  be  seen  the  blood  vessels  which  enter  the  eye  along 
with  the  nerve  to  spread  themselves  over  the  retinal  field.  In 
inflammatory  conditions  of  the  brain,  and  intracranial  tension, 
the  disc  is  crowded  with  engorged  vessels  around  its  edge, 
which  are  raised  above  the  level  of  the  nerve-head — a  condi- 
tion known  as  choked  disc.  In  optic  atrophy,  on  the  contrary, 
the  nerve  substance  wastes  away,  showing  a  depression  in 
the  disc  and  the  lamina  cribrosa  is  plainly  seen.  In  optic 
neuritis  the  disc  is  reddened  and  the  vessels  enlarged.  In 
glaucoma  the  depression  is  exaggerated    into     a     cup,     (See 


140  DISCS,  MASON'S  PUPIL 

Cupped  Disc)  and  the  surrounding  vessels  are  bent  as  they 
emerge.  1  hus  many  important  diseased  conditions  are  made 
manifest  in  the  appearance  of  the  optic  disc. 

Optically  the  disc  has  little  value.  It  is  possible,  by  means 
of  the  ophthalmoscope,  using  the  disc  as  an  objective,  to  esti- 
mate the  refraction  of  the  eye;  but  nobody  would  ever  think 
of  utilizing  so  difficult  and  faulty  a  method.  (See  Ophthalmo- 
scope). The  fact  that  the  disc  is  impervious  to  light  stimula- 
tion makes  it  a  convenient  spot  upon  which  to  focus  the  light 
of  the  retinoscope,  ophthalmoscope,  and  other  brightly  illum- 
inated instruments.  We  also  use  the  diameter  of  the  disc  as 
a  rough  standard  of  measurement  for  other  areas  of  the  retina, 
saying  "about  two  discs  from  the  macula,"  etc. 

Discs,  Mason's  Pupil.  Opacjue  discs,  with  cental  aperture,  of 
various  sizes,  for  the  purpose  of  measuring  the  size  of  the 
pupil  at  rest  and  in  varying  degrees  of  accommodation. 

Discs,  Volkmann's.  A  device  for  determining  the  angular  rela- 
tion of  the  ^■ertical  meridians  of  the  eyes.  Two  revolving 
discs,  each  containing  a  single  radius,  are  pinned  on  the  wall, 
separated  ])y  the  pupillary  width,  and  viewed  steroscopically. 
The  discs  are  then  turned  until  the  two  radii  form  one  contin- 
uous straight  line. 

Discs,  Walker's  Color.  Two  discs,  of  different  color,  aflfixed  one 
to  each  end  of  a  rod,  but  with  their  planes  at  right  angles  to 
each  other,  so  that  it  is  impossible  to  see  both  colors  at  once. 
Used  as  color  tests. 

Disparate  Points.  Objectively  this  term  denotes  two  points,  one 
in  each  retina.  ui)on  which  light  falling  from  the  same  point 
of  an  object  produces  a  double  image.  Subjectively,  it  is  used 
to  denote  the  two  points  in  space  wIumc  these  [\\u  images 
appear  to  be  situated.  The  ajjparenl  separation  i>|  tlu-  two 
images  in  diploi)ia  is  called  their  disparatiDU. 

Dispersing  Lens.  A  lens  which  eausi-s  li.^ht  \\a\es  to  expand, 
and  thus  to  travel  away  from  their  centre,  i.  e.  a  concave  lens. 

Dispersion.  A  term  applied  to  the  anj^uiar  separation  of  the 
coin|»oneni  rays  of  a  pencil  of  light  on  inier_<;ing  from  a  retract- 


DISPLACED  MACULA  141 

ing  medium  whose  surfaces  are  not  parallel  to  each  other,  e.  g. 
a  common  prism.  The  length  to  which  they  are  drawn  out 
varies,  of  course,  with  the  refracting  index  and  surface-inclin- 
ation of  the  dispersing  medium.     (See  Prism). 

Displaced  Macula.  A  macula  which  does  not  occupy  the  posi- 
tion of  visual  centration  in  the  retina.  It  is  a  congenital  condi- 
tion, and  one  of  the  frequent  causes  of  vertical  heterotropia. 

Distance.  The  measurements  of  space  between  two  objects.  In 
optics  the  word  usually  applies  to  the  space  between  two 
fixed  mathematical  points. 

Focal  Distance.  The  distance  between  the  optical  centre  of 
a  lens  or  mirror  and  its  principal  focal  point. 

Infinite  Distance.  Optically,  this  denotes  the  distance  in 
which  waves  of  light,  originating  at  a  luminous  point,  are 
rendered  neutral  or  parallel.  Actually,  of  course,  such  a  dis- 
tance does  not  exist ;  in  practice,  however,  we  regard  waves 
which  have  traveled  6  meters  or  20  feet  as  being  neutral,  and 
therefore  call  6  meters  or  20  feet  an  infinite  distance. 

Pupillary  Distance.  The  distance  between  the  centres  of 
the  two  pupils.    Also  called  the  pupillary  width. 

Distance,  Judgment  of.    See  Physiology  of  Vision. 

Distichia.  Rubbing  of  a  double  row  of  eyelashes  (on  one  lid) 
against  the  cornea. 

Distortion.  A  phase  of  aberration  in  which  the  linear  dimen- 
sions of  the  image  do  not  bear  the  same  proportion  to  each 
other  as  those  of  the  object.  It  is  the  opposite  condition  of 
being  orthoscopic. 

Divergence.  This  term  is  applied  to  two  straight  lines  or  paths 
which  get  continually  further  and  further  from  each  other. 
Such  lines,  or  paths,  if  traced  backward,  would  meet  some- 
where at  a  common  point,  and  this  point  could  be  made  the 
centre  of  a  circle  or  sphere  of  which  they  would  be  radii. 
Divergent  light  waves  are  those  which  are  traveling  away  from 
a  point  of  origin. 

Physiologically,  divergence  signifies  the  swinging  of  the 
visual  axes  outward,  so  as  to  diverge  from  the  middle  line,  by 


142  DIVERGENT  STRABISMUS 

means  of  the  external  rectus  muscles.  Strictly  speaking,  diver- 
gence denotes  only  positive  divergence  beyond  the  middle  line. 
Turning  outward  from  convergence  to  parallelism  is  negative 
convergence. 

Divergent  Strabismus.  Strabismus  in  which  the  visual  axes  in- 
cline away  from  each  other. 

Donders.  A  famous  Dutch  ophthalmologist,  (1818-1880),  who 
contributed  very  extensively  and  originally  both  to  ophthal- 
mology and  also  to  optics.    See  History  of  Optics. 

Bonder's  Glaucoma.     Simple  atrophic  glaucoma. 

Donder's  Laws.  (1)  The  age  at  which  asthenopia  begins  is 
approximately  ecjual  to  the  denominator  of  the  fraction  expres- 
sing the  degree  of  hyperopia.  (2)  The  rotation  of  the  eye- 
ball is  determined  by  the  distance  of  the  point  of  fixation  from 
the  median  plane  and  the  line  of  the  horizon. 

Donder's  Rings.  The  circles  of  yellow  seen  around  lights  in 
glaucoma. 

Donder's  Test.  A  test  for  color  blindness  made  with  lan- 
terns with  colored  slides. 

Double  Cones.     Cones  of  the  retina  which  occasionally  occur  in 

})airs. 

Double  Image  Prism.  Two  right-angled  prisms  cemented 
together  so  as  to  form  a  prism  of  rectangular  section,  lu  split 
a  ray  of  light  into  two  divergent  rays. 

Double  Prism.  A  device  invented  by  Maddox,  consisting  of  a 
prism  of  175  degrees  so  worn  that  the  apex  bisects  the  jnipil 
and  produces  monocular  diplopia,  used  for  testing  muscular 
balance. 

Double  Refraction.  The  si)litting  of  a  ray  of  light  by  refrac- 
tion thrijuf^h  certain  crystals,  l^ei-  Diffraction  and  Polariza- 
tion. 

Double  Vision.  Seeing  two  images  of  the  same  object.  .See 
Diplopia. 

Doublet.  Two  lenses  in  series,  i.  e.  one  jieliiml  the  other  wilii 
iheii-  i)rincipal  axes  coinciding. 


DRIVER'S  TEST  143 

Driver's  Test.  A  test  for  malingering,  to  establish  the  existence 
of  binocular  vision.  By  means  of  a  bar  placed  at  a  certain 
distance  between  the  patient  and  the  test  type,  if  the  patient 
is  able  to  read  binocularly,  the  test  demonstrates  at  once  the 
presence  of  vision  in  the  pretended  blind  eye  and  the  visual 
acuity  which  that  eye  possesses. 

Drops.    A  common  name  for  a  collyrium. 

Duane's  Test.  A  test  for  muscular  imbalance.  See  Hetero- 
phoria. 

Duct.  A  tube  or  passage  for  the  conveyance  of  fluid,  as  the 
tear  duct. 

Duction.  The  power  of  the  various  pairs  of  extrinsic  ocular 
muscles  to  perform  their  functions.  Duction  tests  are  for 
determining  this  power.    (See  Adduction  and  Abduction.) 

Dura  Mater.  The  external,  fibrous  membrane  which  covers  and 
encloses  the  entire  spinal  cord  and  brain.  This  covering  is 
extended  along  the  optic  nerve,  and  is  continuous  with  the 
capsule  of  Tenon  which  surrounds  the  eye. 

Dural  Sheath.  The  continuation  of  the  dura  mater  which  sur- 
rounds and  serves  as  a  sheath  for  the  optic  nerve. 

Dynamic  Refraction.  The  refraction  of  the  eye  with  the  ciliary 
muscle  in  action,  i.  e.  with  some  accommodation  in  efifect.  The 
theory  of  this  method  is  that  when  the  accommodative  effort 
is  in  play  the  ciliary  muscle  will  more  readily  accept  the  help 
of  a  lens  and  surrender  its  spasmodic  contraction,  thus  enab- 
ling us  to  correct  latent  hyperopia  more  fully.  The  working 
principle  is  to  ascertain  both  the  subjective  and  the  dynamic 
findings,  and  the  difference  between  the  two  represents  the 
tonic  contraction  of  the  ciliary.  Dynamic  refraction  is  carried 
out  by  means  of  the  retinoscope,  and  further  details  of  the 
procedure  will  be  found  under  Retinoscopy. 

Dynamic  Skiametry.    See  Retinoscopy. 

Dynanometer.  An  instrument  for  estimating  the  magnifying 
power  of  a  lens. 


144  DYSLEXIA 

Dyslexia.  Difficulty  in  reading,  due  to  impairment  of  the  brain. 
See  Alexia. 

Dysopsia.     Dysopia.     Impaired  vision. 

Eccentric.  Off-centre.  Geometrically  it  is  applied  to  those  forms 
which  are  derived  from  the  circle,  or  sphere,  but  which  are  no 
longer  circular  or  spherical, — the  parabola,  for  example.  In 
mechanics,  it  denotes  a  form  of  power-transmission  in  which 
the  movement  is  changed  from  a  circular  to  a  parabolic  or  a 
side-to-side  movement. 

Ecchymosis.  Extravasation  of  blood  into  a  tissue,  due  to  rup- 
turing of  tiny  capillaries.  It  gives  the  appearance  of  a  dirty 
bluish-yellow.  What  is  commonly  called  a  "black  eye"  is  an 
example  of  ecchymosis.     (X.  B.  It  is  pronounced  En-ki-mosis). 

Echelon  Grating.  A  series  of  gratings,  in  echelon  formation,  for 
the  diffraction  of  light,  devised  by  Michelson,  of  Chicago. 

Echophotony.     Color  sensation  produced  by  aerial  waves. 

Ectasia.     Distention  of  any  organ  or  part. 

Ectiris.     External  part  of  the  iris. 

Ectochoroidea.     ( )uter  part  of  the  choroid. 

Ectocornea.     Outer  layer  of  the  cornea. 

Ectopia  Pupillae.    A  displacement  of  the  pupil. 

Ectoretina.     External  layer  of  the  retina. 

Ectropion.  Outward  eversion  of  the  eyelid.  Usually  affects 
the  lower  lid.  It  interferes  with  closing  the  eye.  and  breeds 
infection  of  the  conjuncti\a.    Operation  is  the  only  cure. 

Edema.  Swelling  of  the  tissues  due  td  accumulation  of  fluid, 
usually  serum  in  the  interspaces,  lulema  of  the  eyelids  is 
usually  a  symptom  of  kidney  or  jieart  disease,  or  else  is  due  to 
a  l)l(»w  or  injury  which  ruptures  the  small  vessels  (black  eye). 

Effective  Rays.  Those  rays  of  light,  out  of  a  pencil,  which  pass 
through  a  refracting  medium  and  fall  ujion  the  screen. 


EFFERENT  145 

Efferent.  Literally,  the  word  means  carrying  outward.  In  phy- 
siology it  is  applied  to  those  nerve  tracts  which  carry  impulses 
from  the  centres  (brain  or  cord)  to  the  peripheries  of  the 
body.  They  are  also  called  centrifugal  tracts.  All  the  motor 
and  secretor  nerves  belong  in  this  class. 

Egilops.     Another  name  for  a  lachrymal  fistula. 

Eidoptometry.     Perception  of  form. 

Edridge-Green  Theory,    A  theory  of  color  vision.     See  Color. 

Elephantiasis  Oculi.     Extreme  exophthalmia. 

Embolism.  The  blocking  of  a  blood  vessel  by  a  clot  or  plug  of 
some  sort  floating  in  the  circulation.  In  the  case  of  an  import- 
ant artery,  the  function  of  the  part  supplied  by  the  plugged 
vessel  abruptly  ceases. 

In  ophthalmology  the  most  serious  embolism  is  that  of  the 
central  retinal  artery,  which  causes  sudden  and  complete 
blindness.  The  affected  retinal  field  becomes  white,  and,  if 
the  embolism  is  not  relieved,  optic  atrophy  ensues. 

Embolus.    The  clot  of  blood  which  causes  embolism. 

Emergent  Ray.  A  ray  of  light  that  emerges  from  a  refracting 
medium,  having  been  acted  upon  by  it. 

Emission  Theory.  .The  corpuscular,  or  Newtonian,  theory  of 
light. 

Emmetropia.  A  normal  state  of  refraction  of  the  eye,  in  which, 
with  the  eye  at  rest,  (i.e.  no  accommodation  in  force),  the 
principal  focal  point  of  the  refracting  system  of  the  eye  lies 
exactly  in  the  plane  of  the  retina,  so  that  neutral  waves  of 
light  are  focussed  on  the  retina.  While  emmetropia  is  the 
normal  condition,  it  is  by  no  means  a  common  one,  completely 
emmetropic  eyes  being  in  a  great  minority. 

Emphysema.  Air  in  the  tissues.  Emphysema  of  the  eyelids  is 
practically  always  due  to  wounds  of  the  orbit. 

Encanthis.    A  small  tumor  in  the  inner  canthus. 


146  ENDOTHELIUM 

Endothelium.  A  membrane,  composed  of  flat  cells,  lining  the 
interior  of  serous  and  other  cavities.  It  is  to  the  interior  what 
epithelium  is  to  the  exterior.  The  fifth  stratum  of  the  cornea 
is  endothelium. 

Enophthalmos.     Recession  of  the  eye  into  its  orbit. 

Enstrophe.     A  turning  inward. 

Entochoroidea.     Inner  la}er  of  the  choroid. 

Entocornea.     Inner  layer  of  the  cornea. 

Entoptic.  W  itliin  the  eye.  In  optics  the  word  usually  refers  to 
subjective  abnormalities  of  vision  which  are  due  to  physical 
bodies  inside  the  eyeball,  ^^'hat  are  known  as  "floating 
specks"  are  entoptic  phenomena. 

Entoptoscopy.     Examination  of  the  interior  of  the  eye. 
Entoretina.     Inner  layer  of  the  retina. 

Entropion.  Turning  inward  of  the  eyelids.  This  is  a  serious 
condition  because  the  lashes  scratch,  and  even  grow  into,  the 
cornea,  making  it  opaque  and  destroying  the  vision.  Opera- 
tion is  the  only  cure. 

Enucleation.     Removal  of  the  entire  c}eball. 

Ephidrosis.     lCxcessi\e  sweating  of  the  eyelids. 

Epicanthus.  A  fold  of  skin  extending  from  the  root  of  the  nose 
to  the  inner  angle  of  the  eyebrow,  overlapping  the  inner  can- 
thus.     It  is  normal  in  Mongolians. 

Epiphora.  .\n  o\  erllow  (jf  tears.  This  occurs  temporarily  under 
any  strong  stimulus  to  the  conjunctixa,  such  as  a  cold  wind. 
or  a  physical  irritation.  I'atliologically  it  is  general)}-  due  to 
stoppage  of  the  tear  duct. 

Episcleral.     !~^ituated  o\  or  the  sclera. 

Episcleritis.  Inll.iinniation  of  the  outer  coat  of  the  sclera.  It 
usually  appears  in  tlie  form  of  a  bulging  spot  of  inflamed  hard- 
ening upon  the  sclera. 


EPITHELIOMA  147 

Epithelioma.  A  form  of  cancer  (carcinoma)  which  has  its  rise 
in  the  cells  of  epithelium.  It  frequently  attacks  the  eyelids, 
beginning  usually  at  the  junction  of  the  lid  and  conjunctiva. 

Epithelium.  The  non-vascular  layer  of  cells  which  covers  the 
exterior  of  skin  and  mucous  membranes. 

Equator.  A  line  drawn  round  a  globular  body  equi-distant  from 
the  poles  at  every  point. 

Equator  of  the  Crystalline  Lens.  The  peripheral  margin  of 
the  lens  which  is  inserted  into  the  zonula. 

Equator  of  the  Eye.  The  poles  of  the  eyeball  are  considered 
to  be  the  points,  anterior  and  posterior,  where  the  principal 
axis  cuts  the  circumference.  The  equator  of  the  eye,  therefore, 
is  the  meridian  mid-way  between  these  two  points  and  at 
right  angles  to  the  axis. 

Equilibration.  An  operation  on  the  extrinsic  muscles  of  the 
eyes,  for  the  purpose  of  equalizing  their  action  and  securing 
orthophoria. 

Equivalent  Lens.  A  single  lens  which,  placed  at  a  certain  fixed 
point  or  distance,  makes  upon  a  screen  the  same  sized  image 
as  a  series  of  two  or  more  lenses  at  dififerent  distances. 

Erect  Image.  A  virtual  image,  such  as  one  gets  of  the  fundus  in 
the  direct  method  of  ophthalmoscopy. 

Erecting  Prism.  A  prism  interposed  in  a  refracting  or  reflect- 
ing system  for  the  purpose  of  rendering  an  inverted  image 
erect. 

Errors  of  Refraction.  Conditions  of  the  eye  which  prevent  the 
single  focussing  of  neutral  waves  upon  its  retina  when  the  eye 
is  at  rest.  There  are,  in  reality,  but  three  forms  of  such  error : 
(1)  Hyperopia,  in  which  the  principal  focal  point  lies  behind 
the  retinal  plane ;  (2)  Myopia,  where  it  lies  in  front,  and  (3) 
Astigmatism,  where  the  eye  has  unequal  refracting  power  in  its 
various  (usually  two)  meridians,  so  that  there  are  more  than 


148  ERYTHROPSIA 

one  principal  focus.     Other  errors  are  combinations  of  these 
three. 

Erythropsia.     A  condition  in  which   everythine^  is  seen  red. 

Eserine.  An  alkaloid  of  the  calabar  bean,  which,  when  put  into 
the  eye,  causes  spastic  contraction  of  the  ciliary  muscle  and 
the  concentric  muscles  of  the  iris,  thus  producing  contraction 
of  the  pupil  (myosis).  It  is  employed  in  1  per  cent  solutions 
of  the  sulphate.  Its  chief  use  is  in  glaucoma,  to  lessen  intra- 
ocular tension.  It  is  sometimes  employed  to  counteract  the 
effects  of  atropine,  but  is  not  very  efifectual.  It  is  also  called 
physostygmine. 

Esophoria.  Tendency  of  the  eye  to  turn  inward.  See  Hetero- 
phoria. 

Esotropia.     Inward  scpiint.     See  Strabismus. 

Eucaine.  A  synthetic  anesthetic  resembling  cocaine,  used  in 
2%  solution  in  the  eye. 

Eversion.     Turning  outward  of  the  eyelid. 

Excavation.  Another  term  for  cupping  of  the  disc.  See  Cupped 
Disc. 

Exocataphoria.  A  combination  of  an  outward  and  a  downward 
imbalance  of  the  eye-muscles.   See  Heterophoria. 

Exophoria.  Tendency  of  the  eye  to  turn  outward.  See  Hetero- 
phoria. 

Exophthalmic  Goitre.  A  f(jrni  of  j^tjitre  in  which  the  eyeballs 
are  pushed  prominently  forward  from  their  sockets.  Accom- 
panying this  symptom  is  usually  another  one,  viz.,  that  the 
upper  lids  do  not  follow  the  downward  movement  of  the  eye 
(Graefe's  sign).  Except  for  these  symptoms,  the  disease  is  not 
properly  an  eye  condition,  ])ut  a  systemic  one.  due  to  degenera- 
tion of  the  th}  roid  gland. 

Exophthalmos.  .Abnormal  protrusion  of  the  eyeball  outward 
and  f(jrward.  It  most  fre(|UfntIy  accompanies  ccrlaiu  forms 
of   goitre.     (  l'"xo])hthalniic    (ioitre.)       There    is   oftiu    apparent 


EXORBITISM  149 

exophthalmos  in  high  myopia  and  conical  cornea;  but  these 
appearances  are  dne  to  the  size  of  the  ball  and  the  bulging  of 
the  cornea,  not  to  pushing  forward  of  the  ball. 

Exorbitism.     Protrusion  of  the  eyeball. 

Exotropia.     Outward  squint.     See  Strabismus. 

Extorsion.     Rotation  outward  of  the  eyeball. 

Extiinsic.  Belonging  to  the  exterior.  The  extrinsic  muscles  of 
the  eye  are  those  which  are  concerned  with  the  movement  of 
the  eyeball.     See  Muscles. 

Eye.  The  organ  of  vision,  as  a  whole.  Aside  from  its  optical 
aspects,  it  is  made  up  of  three  tunics,  or  coats:  (1)  the  nervous 
tunic,  consisting  of  the  optic  nerve  and  its  out-spreading  into 
the  retina,  (2)  the  vasculo-muscular  coat,  comprising  the 
chorioid,  ciliary  body  and  iris,  and  (3)  the  supporting  coat,  or 
sclera. 

The  entire  globe  of  the  eye  is  almost,  but  not  quite,  spherical 
in  shape,  the  antero-posterior  diameter  being  slightly  the  great- 
est, because  of  the  projection  of  the  cornea.  The  diameters  of 
the  adult  eye  are  about  as  follows,  in  millimeters: 

Antero-posterior   24.3 

Transverse   23.6 

A^ertical  23.3 

Depth  of  anterior  chamber 11.9 

Thickness  of  lens  at  rest Z.7 

Thickness  of  lens  at  maximum 4.3 

Considered  as  an  optical  instrument  the  eye  is  a  series  of 
semi-submerged  lenses,  whose  radii  of  curvature  and  indices  of 
refraction  respectively  are  as  follows,  with  air  as  the  index 
base: 

Anterior  surface  of  cornea 7.5  mm. 

Substance  of  cornea 1.333 

Aqvieous  humor   1.336 

Anterior  surface  of  lens 10      mm. 

Substance  of  lens    1.43 

Posterior  surface  of  lens 6      mm. 

Vitreous  humor 1.339 


150  EYE 

The  posterior  surface  of  the  cornea  is  omitted,  as  its  action 
is  negligible ;  the  cornea  and  aqueous  humor,  having  so  nearly 
the  same  refractive  indices,  may  be  regarded  as  one  continuous 
medium. 

With  these  data,  the  values  of  the  respective  surfaces  work 
out  approximately  as  follows : 

Anterior  cornea  .33  of  -(-1^^       =  44  D. 
Anterior   lens       .10  of  +100  D.  =  10  D. 
Posterior    lens      .10  of  +160  D.  =  16  D. 
making  a  total  of  70  D.     Deducting  from  this  figure  12  D.,  at- 
tributable to  the  separation  of  the  refracting  surfaces  and  the 
slight  minus  action  of  the  posterior  cornea,  gives  a  net  dioptric 
power  to  the  eye  of  approximately  58  D.     The  location  of  this 
value,  or  the  point  at  which  a  single  thin  lens  might  represent 
the  entire  dioptric  system  of  the  eye  is  at  the  posterior  nodal 
point,  .4764  mm.  anterior  to  the  posterior  surface  of  the  crystal- 
line lens. 

The  optic  axis  of  the  eye  pierces  the  center  of  the  cornea, 
normal  to  its  curvature,  and  passes  through  the  geometric 
center  of  the  eye-ball.  Situated  along  this  axis  are  three  sets 
of  cardinal  points,  one  set  for  each  refracting  surface.  For  the 
sake  of  convenience,  however,  we  merge  them  into  one  set, 
using  as  a  basis  a  single  refracting  surface  having  the  same 
net  dioptric  power  and  principal  focal  point  as  the  compound 
system — known  as  the  reduced  e}c.  These  points  will  be 
found  to  be  as  follows: 

The  principal  point,  where  the  imaginary  surface  referred 
to  cuts  the  axis,  2.3448  mm.  behind  the  anterior  surface  of  the 
cornea. 

The  princijjal  antericjr  focus,  12.832()  nun.  in  front  of  the 
same  surface. 

The  posterior  priiKii)al  focus,  22.()47  mm.  bcliinil  the  same 
surface. 

The  nodal  point,  .4764  mm.  in  front  of  the  posterior  surface 
of  the  crystalline  lens. 

'ihe  radius  of  this  imaginary  surface  would  be  .^.1248  mtn. 

The  center  of  rotation,  around  which  tlie  eyeb.ill  rotates,  is 


EYEBROWS  151 

a  matter  of  some  dispute,  but  is  probably  13.5  mm.  behind  the 
anterior  surface  of  the  cornea,  also  on  the  principal  axis. 

The  eye  has  various  other  axes  and  angles,  all  of  which  are 
described  under  the  headings  Axis  and  Angle. 

The  iris,  with  its  power  of  contraction  and  expansion,  serves 
as  a  cut-off,  to  protect  the  retina  from  excessive  light,  and  to 
insure  against  peripheral  spherical  aberration.  The  retina  is 
the  sensitive  film  of  the  camera ;  and  the  sclera  is  the  dark 
box,  maintaining  its  hollow,  globular  shape.  (For  further  de- 
tails see  Anatomy  of  the  Eye.) 

Eyebrows.  Two  projecting  arches  of  skin  over  the  upper  borders 
of  the  orbits,  covered  with  short  bushy  hair,  which  serve  to 
protect  the.  eyes  from  above. 

Eyelashes.  Soft,  silky  hairs  that  grow  upon  the  margins  of  the 
upper  and  lower  eyelids.  They  protect  the  eyes  from  the  out- 
side. The  follicles  which  form  the  roots  of  these  lashes  not 
infrequently  become  infected,  and  give  rise  to  what  are  known 
as  styes. 

Eyelids.  Folds  of  external  skin  which  push  their  way  over  the 
eyeball  to  cover  and  protect  it,  above  and  below.  The  boun- 
daries of  the  upper  lids  are  formed  by  the  eyebrows,  but  the 
lower  lids  pass  imperceptibly  into  the  skin  of  the  cheek.  The 
space  made  between  the  two  lids,  when  opened,  is  called  the 
palpebral  fissure. 

The  skin  covering  the  eyelids  is  about  the  thinnest  in  the 
body.  Beneath  the  skin  are  the  muscles  by  which  the  lids  are 
moved  (the  orbicularis  palpebrarum  and  the  levator  palpebrae 
superioris),  and  the  cartilaginous  substance  which  gives  them 
shape  (the  tarsus).  On  the  inner  surface  the  lids  are  lined 
with  reflected  conjunctiva,  under  which  are  the  Meibomian 
glands  and  sebaceous  glands,  both  of  which  secrete  lubricant 
for  the  lids.  Along  the  margins  of  the  lids  are  hair  follicles 
from  which  the  lashes  grow. 

For  minute  anatomical  details  of  the  lids  a  work  on  anatomy 
must  be  consulted.  Optically,  they  are  merely  shutters,  for 
preventing  light  from  entering  the  eye. 


152  EYEPIECE 

Eyepiece.  The  lens,  or  combination  of  lenses,  at  the  end  of  an 
optical  instrument,  such  as  a  telescope,  microscope,  etc.,  where 
the  observer's  eye  is  applied. 

Eye,  Schematic.  An  artificially  constructed  model  of  the  eye, 
so  constructed  as  to  serve  as  a  model  upon  which  the  student 
can  practice  retinoscopy  and  ophthalmoscopy. 

Facial  Paralysis.  Paralysis  of  the  seventh  cranial  nerve,  which 
supplies  all  the  muscles  of  the  face  and  the  orbicularis-  palpe- 
brarum on  the  same  side  as  the  nerve.  Not  only  is  it  impos- 
sible to  move  the  paralyzed  side  of  the  face,  but  the  paralyzed 
muscles  become  entirely  flaccid,  so  that  all  the  creases  and 
wrinkles  of  the  face  are  smoothed  out.  So  far  as  the  eye  is 
concerned,  the  important  thing  is  that  the  lid  cannot  be  closed ; 
the  eye  remains  permanently  open,  and  soon  becomes  in- 
flamed and  the  cornea  dulled  from  constant  exposure. 

Facultative.  This  term  is  api)lied  to  a  physiologic  disability 
under  which  the  patient  is  still  able  to  function  normally.  Thus, 
facultative  hyperopia  is  a  condition  of  hyperopia  in  which  the 
patient  can  still  exercise  clear  distant  vision. 

False  Image.  The  image  made  upon,  and  seen  by,  the  dc\  iating 
eye  in  genuine  strabismus.  Tlic  image  is.  of  course,  really  no 
more  false  than  that  of  the  sound  eye.  but  is  projectcil  to  a 
false  position. 

False  Myopia.  Where  a  person  has  a  spasm  of  the  ciliary 
muscle,  the  crystalline  lens  being  thus  kept  continually  in  an 
excessively  convex  cur\  riturc,  the  condition  simulates  myopia, 
and  is  sometimes  mistaken  for  it,  and  careless  practitioners  not 
infref|uently  give  such  patients  minus  glasses.  This  is  known 
as  false  myopia. 

False  Projection.  W  lun  light  from  an  object  has  bien  de\iatod. 
as  by  a  prism,  before  enteiinL;  the  eye,  the  brain  projects  the 
rays  back  in  a  straight  line,  and  the  object  thus  appears  to  be 
in  a  position  in  space  different  from  that  which  it  actually 
occupies.    This  is  known  as  false  i)rojiction.     (.'^ee  Projection.) 


FAR  POINT  153 

Far  Point.    See  Accommodation  and  Convergence. 

Far-Sight.    A  popular  name  for  hyperopia. 

Fascia.  A  band  of  connective  tissues  covering  and  connecting 
muscles. 

Fatigue-Field.  The  limits  of  the  field  of  vision  found  in  neuras- 
thenics.    See  Perimetry. 

Fereol-Graux  Palsy.  Paralysis  of  the  external  rectus  of  one  eye 
and  the  internal  rectus  of  the  other.     Of  cerebral  origin. 

Field,  Retinal.  The  area  of  the  retina  over  which  the  stimulation 
of  light  produces  the  sensation  of  vision.  This  extends  prac- 
tically over  the  entire  area  of  the  retina,  the  acuity  of  vision 
being  keenest  at  the  fovea  centralis,  and  diminishing  concen- 
trically in  direct  ratio  to  the  distance  from  that  spot.  Varia- 
tions in  shape  and  scope  of  the  visual  area  are  not,  as  a  rule, 
due  to  retinal  conditions  (except  in  disease),  but  to  conditions 
uhich  govern  the  entrance  of  light  into  the  eye,  and  pertain 
to  the  visual  field.  The  whole  subject  will  be  treated  under 
the  heading  of  the  visual  field.     See  Field,  Visual, 

Each  retinal  field  is  divided  into  two  lateral  halves,  known 
as  the  nasal  and  temporal  halves  respectively,  by  reason  of  the 
decussation  of  the  optic  nerve  fibres  from  these  two  halves 
at  the  optic  chiasm. 

Field,  Visual.  The  area  in  space  which  the  vision  is  able  to 
compass.  The  monocular  field  is  the  area  which  one  eye  alone 
can  compass;  the  binocular  field  the  area  which  both  eyes  to- 
gether can  take  in. 

If  waves  of  light  did  not  cross  at  the  nodal  point,  the  visual 
field  would  be  lim.ited  to  the  size  of  the  pupillary  aperture ;  but 
the  crossing  of  the  rays  permits  of  a  visual  area  as  large  as 
the  angular  separation  of  the  two  lines  forming  the  visual 
angle.  The  form  of  the  visual  field  depends  upon  the  way  in 
which  the  light  enters  the  eye,  and  this  is  modified  by  certain 
interruptions  or  cut-offs.     Notably,  the  entrance  of  light  from 


154  FIFTH  NERVE  PARALYSIS 

the  nasal  side  is  considerably  cut  off  by  the  nose,  so  that  the 
visual  field  on  that  side  is  markedly  limited  over  that  on  the 
temporal  side.  The  normal  forms  of  the  visual  fields  for  the 
two  eyes  is  as  shown  in  the  accompanying  cut. 

Physiologically,  the  visual  field  may  be  considered  as  cen- 
tral and  peripheral.  The  central  field  is  represented  by  those 
light  waves  which  fall  upon  the  macula.  These  images  alone 
we  see  with  clearness  and  attention,  and  as  they  fall  on  sym- 
metrical spots  in  the  two  retinae,  these  images  are  exactly 
superimposed  on  each  other  and  appear  as  one.  From  the 
peripheral  field  the  light  falls  on  the  peripheral  parts  of  the 
retina,  making  images  which  are  less  keenly  perceptible  and 
more  or  less  blurred,  being  less  accurately  focussed  and  falling 
upon  less  sensitive  rods  and  cones.  Moreover,  these  images 
are  not  made  on  symmetrical  points  in  the  tw'o  retinae ;  hence 
images  from  the  peripheral  field  are  double,  and  overlap  each 
other.  However,  they  are  good  enough  images  to  protect  us 
as  we  move  around,  and  to  call  our  attention  to  objects  which 
we  wish  to  "fix"  and  to  view  attentively.  Again,  owing  to  the 
interposition  of  the  nose,  there  is  a  certain  portion  of  the  visual 
field  on  each  side  which  is  represented  only  on  the  retinal  field 
of  each  eye,  respectively,  on  the  temporal  side.  The  rest  of  the 
visual  field  overlaps  in  both  retinae. 

The  visual  field  is  exjilorcd  and  measured  by  means  of  an 
instrument  called  the  perimeter  ((|.  v.).  Diseases  of  the  retina 
produce  characteristic  changes  in  the  field,  which  serve  as 
means  of  diagnosis. 

'ihe  differences  in  the  ])eripheral  \  isual  field  of  the  two  eyes, 
i.  e.,  the  different  views  which  each  eye  has  of  the  objects 
which  are  seen,  play  an  important  i)art  in  what  is  ktunvn  as  the 
stereoptic  sense,  i.  e.,  tlic  pt'rce]»tion  of  solidity,  and  are  made 
use  of  in  the  constnulioii  of  stereoiiticon  iiistruineiits.  ( !^ec 
Binocular  Vision.) 

Fifth  Nerve  Paralysis.  The  fifth  cranial  nerve  mediates  sensa- 
tion f(jr  practically  tlu-  I'ulire  head  and  face,  iiuhiding  the  eye- 
ball.    When,  therefore,  the  ophthalmic  branch   is  involveil   in 


FINDER  155 

paralysis  of  the  nerve,  the  winking  reflex  of  the  cornea  is  lost, 
the  eye  becomes  dry,  dust  accumulates  on  the  conjunctiva  and 
the  cornea,  with  resulting  inflammation  and  destruction  of 
corneal  transparency. 

Finder.  A  device  on  any  optical  instrument  which  enables  us 
to  find  the  focus  necessary  to  the  functioning  of  the  instru- 
ment. 

Fissure,  Palpebral.  Opening  between  the  margins  of  the  eye- 
lids. 

Fixation.  In  physiologic  optics  this  term  is  applied  to  the  hold- 
ing of  the  accommodation  and  convergence  of  one  eye  (monoc- 
ular fixation)  or  both  eyes  (binocular  fixation)  for  a  given 
point  of  distance. 

Fixation  Line.  The  line  joining  the  point  of  fixation  with  the 
center  of  motility  of  the  eye  or  eyes. 

Fixation  Test.    Test  of  the  near  point  of  fixation. 

Flap  Extraction.  A  method  of  extracting  a  cataract  by  making 
a  flap  in  the  cornea. 

Floating  Specks.  Small  floating  opacities  in  the  vitreous,  seen 
by  the  patient.    See  Muscae  Volitantes. 

Fluorescence.  The  property  of  rendering  visible,  as  light,  the 
actinic  or  ultra-violet  rays,  or  of  becoming  self-luminous  when 
exposed  to  such  influences. 

Fluorscope.  An  instrument  for  observing  the  efifects  of  non- 
visible  rays  by  means  of  their  action  upon  a  fluorescent  screen. 
Most  commonly  used  in  connection  with  Roentgen  rays. 

Focal.     Pertaining  to  a  focus. 

Focal  Planes.  Plane  surfaces  passing  through  focal  points 
at  right  angles  to  the  principal  axis  of  the  focal  system. 

Focal  Point.  The  point  at  which  a  light  wave  is  brought 
to  a  focus. 


156  FOCUS 

Focal  Distance.  The  distance  from  a  refracting  or  reflect- 
ing surface  to  the  focal  point. 

Focal  Length.  Focal  distance  as  a  property  of  the  lens  or 
mirror.  The  focal  length  is  found  by  di\iding  the  dioptres 
in  1. 

Focus.  The  point  at  which  a  wave  of  light  reverses  itself  from 
a  minus  into  a  plus  wave.  Fvery  wave  of  light  originates  in 
a  point,  and  expands  in  the  form  of  a  sjjhere.  If  nothing. inter- 
feres with  it,  it  will  continue  to  expand  infinitely.  If,  however, 
a  convex  lens  or  a  concave  mirror  of  greater  curvature  than 
the  wave  be  interposed  in  its  path,  the  lens  or  mirror  will  re- 
verse the  curvature  of  the  wave  and  cause  it  to  diminish  to  a 
point  similar  to  the  point  from  which  it  originated.  Passing 
this  point,  it  will  be  again  reversed  into  an  expanding  (plus) 
wave,  and  again  continue  to  expand  until  another  lens  or  mirror 
interrupts  it.  The  point  at  which  this  reversal  takes  place, 
under  the  influence  of  the  lens  or  mirror,  is  called  the  focal 
point,  because  all  of  the  light  contained  in  the  wave  is  there 
concentrated  in  that  point. 

Geometrically,  the  focus  is  represented  by  the  meeting  and 
crossing  point  of  two  or  more  straight  lines  forming  the  radii 
of  the  wave  curve.  This  leads  to  an  erroneous  conception  in 
the  mind  of  some  students  that  the  focal  point  of  a  lens  or  a 
mirror  is  the  point  where  the  refracted  wa\cs  meet  and  cross 
each  other.  That  point  is  the  nodal  point,  which  in  a  spherical 
lens  is  in  front  of  the  focus,  so  that  the  waxes  have  all  crossed 
before  they  are  brought  to  a  focus. 

For  details  of  the  focus  and  its  geometrical  relations,  see 
Lens. 

Fogging.  :\  tccluiical  iianic  gi\eii  to  a  nu-thod  of  testing;  the 
refraction  of  the  eye  by  first  rendering  it  hij^hly  myopic  by 
means  of  a  strong  convex  lens,  and  then  gradually  reducing 
the  plus  power  by  means  of  minus  lenses,  thus  mt)\  ing  the 
light  focus  back  initil  it  reaches  the  retina  and  j^ixes  the  patient 
clear  vision.  The  latter  part  of  the  process  is  known  as  bring- 
ing the  patient  out  of  the  fog. 


FOLLICLE  157 

The  principle  of  the  fogging  system  is  to  force  a  relaxation  of 
the  ciliary  muscle,  so  that  the  testing  may  be  done  with  the 
accommodation  at  complete  rest.  It  takes  the  place  of  the  old 
practice  of  the  ophthalmologist  of  paralyzing  the  ciliary  with  a 
drug.     (See  Hyperopia.) 

Follicle.  Small  secretory  sac.  The  hair  follicles  in  the  eyelids, 
containing  the  roots  of  the  lashes,  and  the  lymph  follicles  of 
the  conjunctiva,  are  ophthalmological  examples. 

Follicular  Conjunctivitis.  A  form  of  conjunctivitis  in  which 
there  are  small  elevated  spots,  or  follicles,  in  the  membrane. 
It  usually  attacks  the  lower  lid.  Errors  of  refraction  aggra- 
vate the  condition. 

Fontana's  Spaces.  The  triangular  spaces  between  the  sclera  and 
the  root  of  the  iris.  They  are  supposed  to  play  a  part  in  the 
causation  of  glaucoma  (q.  v.). 

Foramen.  An  opening  in  bone  for  the  passage  of  vessels,  nerves, 
etc.  The  one  foramen  of  interest  in  ophthalmology  is  the 
Optic  Foramen,  through  which  the  optic  nerve  and  its  accom- 
panying vessels  enter  the  orbit.     (See  Anatomy.) 

Foreign  Bodies  in  the  Eye.  This  term,  as  commonly  employed, 
refers  to  small  particles  of  matter,  such  as  stone,  metal,  cinder, 
emery,  etc.,  which  become  lodged  in  the  conjunctival  sac  or 
on  the  exterior  of  the  eyeball.  They  immediately  set  up  a  very 
violent  irritation,  causing  profuse  flow  of  tears,  redness  and 
swelling  of  the  conjunctiva,  and  more  or  less  acute  pain. 

If  the  foreign  body  is  lodged  in  the  lower  conjunctival  fornix, 
it  can  be  readily  removed  by  drawing  down  the  lower  lid  and 
catching  the  oflfending  particle  on  a  little  piece  of  cotton  twisted 
on  the  end  of  a  toothpick  or  probe.  If  it  be  under  the  upper 
lid,  the  lid  must  be  turned  back, — best  accomplished  by  laying 
a  smooth  probe  or  rod  along  the  upper  part  of  the  eyelid,  where 
it  lies  on  the  eyeball,  telling  the  patient  to  look  down  at  his 
feet,  taking  hold  of  the  lashes,  and  turning  the  lid  over  the 
probe, — and  the  same  method  applied  as  in  the  case  of  the  lower 
fornix. 

If  the  particle  be  of  a  hard  substance  and  is  bedded  in  the 
cornea  its  removal  is  rather  more  difficult.    It  must  be  dug  out 


158  FORNIX  CONJUNCTIVAE 

w  ith  a  small  pick,  for  which  operation  a  Httle  cocaine  is  usually 
needed,  as  the  cornea  is  exquisitely  sensitive.  Great  care  must 
be  taken  not  to  dig  too  deeply,  so  as  to  penetrate  the  corneal 
epithelium  and  wound  the  external  limiting  (Bowman's)  mem- 
brane. 

Usually,  after  a  foreign  body  has  been  successfully  removed 
all  the  inflammatory  trouble  (piickly  subsides  and  the  eye  re- 
turns to  normal  within  a  few  hours.  There  is  always  the  pos- 
sibility, however,  of  infection,  especially  where  the  cornea  has 
been  wounded.  It  is  better  to  follow  removal  from  the  cornea 
by  a  good  flushing  of  the  eye  with  saturated  solution  of  boric 
acid. 

Foreign  bodies  which  penetrate  the  cornea  and  enter  the  in- 
terior of  the  eye  are  a  much  more  serious  affair,  and  belong 
wholly  to  the  sphere  of  the  expert  eye  surgeon. 

Fornix  Conjunctivae.  The  angle  of  the  conjunctixa  where  it  is 
reflected  from  the  bulbar  to  the  palpebral  i)ortion. 

Fossa  Patellaris.  The  concave  depression  in  the  vitreous  body 
in  which  tlie  posterior  surface  of  the  crystalline  lens  lies.  Also 
called  the  Hyaloid  Fossa. 

Four  Dot  Test.  A  test  devised  by  \\ Orth  lor  binocular  \isi<»n. 
It  consists  of  four  circular  holes  in  a  diaphragm,  arranged  in 
diamond  form,  ha\ing  a  red  glass  in  the  top  hole,  green  in  the 
tw(j  lateral  holes,  and  white  in  the  bottom  one.  The  patient 
wears  a  red  glass  before  the  right  eye  and  green  one  before 
the  left.  If  he  sees  two  dots,  black  and  white,  he  is  using  the 
right  eye  only.  If  three  dots,  white  and  two  green,  the  left  eye 
only.  If  four  dots,  he  uses  both  eyes.  If  he  sees  fi\e  dots,  red, 
two  green  and  two  white,  he  has  diplopia. 

Fovea  Centralis.  The  small  depression  in  the  center  of  the  yellow 
s])ot  which  forms  the  keenest  center  of  retinal  acuity.  Sec 
Retina. 

Frame  Fitting.  Tlicre  is  iKtlhing  of  greater  iinptu  t;mce  in  tlie 
lilting  of  glasses  than  fitting  the  frames  or  mountings,  for  if 
the  lenses  do  not  set  pr(»j)erly  before  the  eyes  satisfactory 
results  c.iuiitit   be  CNpccted, 


FRAME  FITTING 


159 


THE  SPECTACLE  BRIDGE. 
There  are  two  ways  of  expressing  the  dimensions  of  a 
bridge :  By  giving  each  dimension  in  figures  or  by  using  the 
size  letter  and  number.  The  dimensions 
considered  are  height,  inclination  of  crest, 
angle  and  width  of  base.  The  following 
letters  are  used  to  designate  the  width  of 
bridges,  beginning  with  the  smallest: 
L,  M,  N,  O,  P.  The  heights  are  expressed 
in  combination  with  the  letters  by  num- 
bers, as  Yz,  1,  XVi,  2,  etc.  The  shanks  are 
called  regular,  long  and  extra  long.  With 
the  regular  shanks  the  lenses  are  held  a 
trifle  closer  to  the  eyes  than  the  crest  of 
the  bridge ;  with  long  shanks  the  lenses 
and  crest  of  bridge  are  on  the  same  plane ; 
with  extra  long  shanks  the  lenses  are 
further  from  the  eyes  than  the  crest  of 
bridge  is.  Thus  to  set  the  lenses  away 
from  the  eyes  to  escape  the  lashes,  etc., 
we  use  long  and  extra  long  shanks.  When 
no  length  shank  is  stated  '"regular"  is 
understood.  This  is  the  way  the  different 
sizes  of  bridges  are  expressed:  M,  M^, 
N2  extra  long  shanks. 

When  the  sizes  are  not  specified  as 
above  it  is  necessary  to  give  all  the  di- 
mensions in  figures.  The  height  of  bridge 
is  the  distance  above  or  below  a  line  run- 
ning through  the  center  of  the  lenses  to 
the  lower  edge  of  the  center  of  bridge ; 
the  inclination  of  the  crest  is  the  distance 
from  the  inside  plane  of  the  lenses  to  the 
upper  edge  of  the  middle  of  the  bridge 
and  is  specified  "in''  or  "out,"  meaning 
in  back  or  in  front  of  the  lenses,  re- 
spectively. The  angle  of  the  bridge  is  considered  with  respect 
to  the  plane  of  the  lenses,  the  latter  being  90  dgrees.  The 
angle  is  measured  at  the  center  or  crest  of  the  bridge. 


DIMENSIONS  OF  SAD- 
DLE BRIDGES. 

(Upper  figure  Inches, 
lower  figure  Millimeters). 

tie 

n 

'35 

to 

V 

U 

n 

L 

0 
0 

0 
0 

16 

LJ-2 

& 

0 
0 

15 

Li 

is 
3 

0 
0 

5i 
IS 

M 

M^2 

0 
0 

VA 

16 

& 

16 

Ml 

4 
3 

tfA 

16 

MIH 

4^ 

1.4 

5-8 

16 

M2 

6 

t„ 

if 

1 

N 

0 
0 

fe 

HM 

iu 

fc 

Nl 

!-8 

3 

& 

18 

NIK 

VA 

18 

N2 

6 

VA 

18 

N23^ 

& 

3 

20 

N3 

r- 

VA 

O 

0 
0 

0 
0 

2L 

Ol 

3 

^A 

21 

03 

6 

& 

21 

03 

r' 

VA 

23 

PI 

3 

fe 

1 
26 

P2 

6 

fc 

1 
25 

P3 

r« 

3 

1 
26 

1 

160 


FRAME  FITTING 


TEMPLES. 
The  length  of  temples  is  measured  from  tip  to  tip,  that  is, 
from  the  screw  hole  to  the  extreme  other  end.     The  average 
length  is  six  inches,  but  they  are  also  made  in  lengths  of  5^, 
6^  and  7  inches. 

SIZES    OF    LENSES. 

Lenses  are  always  measured  in  millimeters,  to  indicate 
length  and  width  as  41  mm.  x  32  mm.  meaning  41  mm.  long 
and  32  mm.  wide.  There  are  certain  standard  sizes  such  as 
000  eye,  00  eye,  0  eye,  38  mm.  oval,  40  mm.  oval,  38  mm.  short 
oval,  etc.  There  are  certain  standard  shapes  such  as  oval, 
short  oval,  drop,  and  round. 

The  following  table  shows  the  dimensions  of  the  standard 
sizes : 

STAha>ARD  SIZES  OF  EDGED  LENSES 


44  mm 

43  mm 

46  mm 

38  mm 

Si/.e 

No. 

Siie 

No. 

Site 

No 

Sue 

No. 

Round 

Ov»l    ...  ., 

44 

48      I  3V 
47.2  1  40.2 
45.7  .  42.2 

440 
449 
447 
443 

42 

46     I  37 
45.2  I  38.2 
43.2  .  40.2 

420 
429 
427 
42i 

40 

44      I  35 
43.2  I  36.2 
41.2  .  38.2 

400 
409 

38 

42      1  33 

380 
389 
387 
383 

FullOv«l. 

407      41.2  I  34.2 
403       39.2  I  36.2 

ooo 

00                                 1 

Siie 

No 

Sue 

No. 

Btvtl 

RimleM 

35.6 

39.7  I  30  7 
38.7  I  31.7 
36.7  ,  33  7 

35.6 

40.     I  31 
40      X  33 
36.7  ,  33.7 

3560 
3869 
3567 
356* 

Ovil                           ^ - 

41      1  32 
39.8  X  32,8 
f.C  1  34.8 

3689 
J687 
3683 

Full  Ovtl     ,         

Round ...- 

Ovil  - 

Full  Ov«l 

0 

1 

Si» 

No 

Sit* 

No. 

Urvel 

RimUit 

Bevel 

Rimlcti 

J7.H  1  28.8 

38.5  I  2^  5 
38.5  1  31  « 

.43  .S9 
3357 

3259 
3257 

:::•:;:■:::::•::■;:: 

36  <  1  27  5 
35  «  .  2M 

37      .  28 
1'      >  30 

Drop  Eye 

" 



'Ihc  cut  on  this  page  illustrates  a  measuring  cartl  used  ft)r 
measuring  spectacle  frames.  Wnw  wholesale  lunise  will 
su])ply  you  with  one  of  these  cards. 

To  measure  I'.  I),  and  height  of  bridge,  place  end  pieces  on 
line  A-A  with  inner  edge  of  left  eye  at  line  IV  The  ligure  at 
right    ciid    of    right    Iciis    indicates    the    pupillary    distaiici-    ;iiul 


FRAME  FITTING 


161 


Protractor. 


that  at  under  edge  of  bridge  crest  indicates  the  height  of 
bridge. 

To  measure  bridge  crest,  forward  or  back,  place  lenses  in 
slots,  top  down,  with  inner  surface  of  lenses  on  lower  edge 
of  slots.  That  edge  of  bridge  resting  on  card  will  indicate 
position  of  crest. 

It  will  be  noticed  that  in  measuring  the  "pupillary  width" 
of  spectacles  and  eyeglasses,  a  similar  plan  is  followed  as  when 
measuring  over  the  eyes ;  that  is,  the  distance  is  taken  from 
the  nasal  edge  of  one  lens  or  rim  to  the  temporal  edge  of  the 
other  lens  or  rim.  This  is  most  conveniently  accomplished  by 
using  the  measuring  card  designed  for  this  purpose  shown 
here. 

THE  PUPILLARY  DISTANCE. 

The  first  thing  for  consideration  is  the  distance  between  the 
pupils.  What  we  desire  to  know  is  the  exact  distance  from  the 
center  of  one  pupil  to  the  center  of  the  other,  but  as  it  is 
impracticable  to  measure  from  these  points  we  measure  from 
the  nasal  side  of  the  right  iris  to  the  temporal  side  of  the  left 
iris  which  gives  the  desired  result.  To  obtain  the  Pupillary 
distance,  or  P.  D.,  as  it  is  generally  termed,  for  glasses  for 
general  use,  for  both  near  and  far,  have  the  patient  fix  his  eyes 
on  an  object  a  couple  of  feet  in  back  of  you  and  with  a  milli- 
meter rule  in  your  right  hand  proceed  to  measure.  First  close 
your  right  eye  and  sight  the  zero  of  your  rule  over  the  nasal 
edge  of  the  patient's  right  iris,  then  close  your  left  eye  and 
open  your  right  and  with  it  read  the  figure  on  the  rule  directly 
opposite  the  temporal  side  of  the  patient's  left  iris.     To  get 


162  FRAME  FITTIMG 

the  correct  distance  between  the  centers  of  the  lenses  for 
reading  distance  close  one  eye  and  direct  your  patient  to  look 
into  your  open  eye  and  make  all  your  readings  with  this  one 
eye.  In  the  latter  case  your  eye  should  be  at  12  or  14  inches 
in  front  of  your  patient's  eyes. 

DETAILS  FOR  SPECTACLES. 

Before  starting  to  ascertain  the  correct  sizes  for  spectacles 
you  should  be  provided  with  a  rule  graduated  in  millimeters 
and  inches,  one  that  is  about  six  or  six  and  a  half  inches  long. 
It  is  preferable  to  make  all  measurements  in  millimeters  as 
this  method  is  much  simpler  and  there  is  less  likelihood  ofi 
making  errors.  Temples,  however,  are  regularly  expressed  in 
inches,  such  as  6  x  6^^,  and  7  inches,  therefor  the  need  of  the 
inch  scale.  You  also  need  a  complete  fitting  set  of  spec- 
tacles. 

After  measuring  the  P.  D.  as  already  explained  select  from 
the  fitting  set  the  spectacle  frame  having  a  bridge  that  comes 
nearest  to  fitting  the  patient's  nose  and  note  the  letter  and 
figure  that  represents  this  size.  If  there  is  no  regular  stock 
size  of  bridge  that  will  fit  the  patient  it  will  then  be  necessary 
to  measure  so  as  to  give  the  exact  specifications  for  height, 
width  of  base,  position  of  crest  and  angle  of  crest,  but  most 
cases  can  be  satisfactorily  fitted  from  the  fitting:  set. 

LENGTH  OF  TEMPLES. 

There  are  two  ways  of  expressing  the  length  of  temples 
desired,  i.  e.,  the  distance  to  back  of  the  ear  or  the  entire 
length  of  the  temple  from  tip  to  tip.  The  first  measurement  is 
made  with  the  fitting  spectacles  on  the  patient's  face,  the  two 
extreme  points  being  the  plane  of  the  lenses  antl  the  miildle 
of  the  back  of  the  ear.  The  other  method  is  to  notice  how 
the  length  of  the  temples  on  the  fitting  frame  suits,  measuring 
the  full  length  of  these  temples  and  then  adiling  to  or  sub- 
tracting from  this  length  as  may  be  necessary. 

The  instructions  given  here  apply  to  both  rimless  and 
frames.  Some  use  four  or  fi\e  spectacles  of  difi'erent  sizes  to 
measure  over,  but  the  use  of  a  complete  set  of  IJ  sizes  is 
strongly  advised. 


I 


FRAME  FITTING  163 

EYEGLASSES. 
The  finger-piece  type  has  come  into  use  within  the  last 
ten  years  and  on  account  of  neatness  of  appearance,  the  prop- 
erty of  retaining  its  original  shape  and  adjustment,  and  sim- 
plicity in  fitting,  it  has  become  very  popular  and  widely  used. 
However,  there   are  cases   where   the   regular  style   is   more 


Finger-Piece.  Regular. 


desirable  than  the  finger-piece  and  vice  versa.  For  instance,  a 
finger-piece  mounting  has  a  tendency  to  cause  the  nose  to 
appear  shorter  and  the  face  narrower,  while  the  regular 
mounting  gives  rise  to  reverse  impressions.  This  being  the 
case  if  you  put  a  finger-piece  mounting  on  a  short  nose  you 
make  it  seem  shorter ;  a  regular  mounting  would  lengthen  it. 
If  you  fit  a  finger-piece  mounting  where  the  pupillary  distance 
is  comparatively  narrow,  the  eyes  will  seem  still  closer 
together,  whereas  a  regular  mounting  will  seem  to  put  more 
space  between  the  eyes. 

"REGULAR"  STYLE. 

To  ascertain  the  correct  size  of  lens,  length  of  stud,  style  of 
guard,  etc.,  it  will  be  quite  necessary  to  have  an  eyeglass 
mounting  to  measure  over. 

First  measure  the  patient's  P.  D.  Then  adjust  your  sample 
mounting  as  well  as  you  can  and  place  it  in  the  correct  posi- 
tion on  the  patient's  nose.  Now  measure  the  P.  D.  of  the 
glasses  while  on  the  face  (measure  from  inside  edge  of  one 
lens  to  outside  of  the  other)  ;  this  places  you  in  position  to 
know  how  large  to  make  the  lenses  and  how  long  the  studs. 
Suppose,  for  illustration,  that  the  sample  mounting  is  equipped 
with  regular  B  studs  and  0  eye  lenses,  that  your  patient's 
P.  D.  is  60,  and  that  the  P.  D.  of  the  glasses,  when  on,  is  58 
millimeters.  You  see  at  a  glance  that  these  glasses  would  be 
too  narrow  and  their  P.  D.  must  be  increased  2  millimeters. 

There  are  two  ways  in  which  this  can  be  accomplished ;  by 
using  longer  studs  or  larger  lenses.     The  next  size  studs  to 


164  FRAME  FITTING 

those  on  the  sample  mounting  are  known  as  C  studs,  there 
being  a  difference  of  one  millimeter  in  the  length  of  a  B  and  a 
C.  By  using  C  studs  in  the  case  we  are  considering  we  will 
increase  the  P.  D.  of  the  glasses  2  mm.  (1  mm.  on  each  stud), 
and  thus  obtain  the  desired  width  of  60  mm.  By  increasing 
the  size  of  lenses  2  mm.  and  leaving  the  studs  as  they  are  in 
the  sample  (B  size)  we  can  obtain  the  same  result.  The  lenses 
in  our  sample  are  0  eye  size  and  their  length  therefore  is  39 
mm.;  adding  2  mm.  to  this  gives  41,  which  is  the  length  of 
000  eye  lenses,  hence  by  using  000  lenses  and  B  studs  we 
obtain  the  desired  P.  D.  With  these  two  methods  we  can  make 
several  combinations  and  get  exactly  the  dimensions  we  want. 
For  instance,  we  have  studs  ranging  from  A  to  F  (about  1  mm, 
difference  for  each  size)  and  lenses  ranging  from  1  eye  to 
jumbo,  or  in  figures,  from  37  to  46  mm.  long,  which  we  can 
combine  in  a  great  many  different  ways. 

Notice  when  the  mounting  is  in  the  proper  position  on  the 
nose  whether  the  lenses  are  too  close  to  or  too  far  away  from 
the  eyes.  If  they  are  too  close  use  inset  studs  to  put  them 
farther  out,  if  too  far  away  use  outset  studs  to  bring  them 
closer.  Both  of  these  styles  are  made  in  two  sizes,  1-16  and  1-8 
inch,  and  you  can  easily  tell  which  size  is  required. 

If  the  brows  are  prominent  and  press  against  the  spring  use 
a  Grecian  or  a  tilting  spring.  Oblong  springs  are  usually  used 
for  men  and  hoop  springs  for  women,  but  this  is  a  matter  of 
personal  choice. 

The  guards  selected  should  have  a  flat  surface  where  they 
come  into  contact  with  the  flesh — this  is  the  first  requisite  of 
an  efficient  guard.  In  adjusting  the  guards  it  must  be  borne 
in  mind  that  contact  and  adhesion  count  greater  for  desirable 
results  than  pressure,  and  for  this  reason  the  guard  must  be 
curved  and  bent  to  ct)nform  with  the  corrcspoiuling  part  of 
the  nose. 

Yc^u  should  have  about  six  eyeglass  mountings,  complete 
with  lenses,  and  having  difTcrent  styles  of  guards  and  springs. 
With  this  e(|ni])mont  you  can  si-lect  the  style  of  guard  that 
will  be  best  for  each  particular  case. 

Some  styles  and  angles  of  guards  will  set  the  lenses  lower 
than  (fliers,  but  usually  it  is  necessary   to  drill   the  holes  in 


FRAME  FITTING  165 

the  lenses  1-16  or  1-8  inch  above  center  to  lower  them, 
especially  where  the  glasses  are  to  be  bifocal  or  reading  lenses, 
in  regular  eyeglass  mountings. 

FINGER-PIECE  EYEGLASSES. 

You  must  be  provided  with  a  complete  fitting  set  of  some 
good  make  of  mountings.  Do  not  make  the  common  mistake 
of  getting  a  few  mountings  of  several  kinds,  but  get  a  full  set 
of  some  one  particular  style;  if  they  are  good  mountings,  with 
the  proper  adjustment,  they  can  be  made  to  fit  any  nose  that 
could  wear  the  eyeglasses,  and  by  getting  a  full  set  you  have 
the  entire  range  of  numbers  and  sizes  to  select  from. 

With  the  fitting  set  at  hand,  select  the  mounting  that  comes 
nearest  to  fitting,  take  your  pliers  and  adjust  the  mounting  so 
that  it  will  assume  just  about  the  same  position  that  the 
mounting  you  order  will  when  adjusted.  Some  manufacturers 
do  not  advise  adjusting  the  mountings  in  the  fitting  set,  but 
experience  proves  that  it  is  better  to  do  this,  for  you  are  then 
in  position  to  know  definitely  whether  the  mounting  can  be 
made  to  fit  or  not,  and  to  accurately  ascertain  the  size  of 
lenses  and  the  kind  of  posts  required. 

Having  decided  what  mounting  fits  the  best,  note  the  num- 
ber it  bears  that  represnts  its  size.  Measure  the  P.  D.  of  the 
patient  and  then  measure  the  P.  D.  of  the  glasses.  If  these 
two  measurements  are  alike  prescribe  the  same  size  lenses  as 
those  in  the  fitting  mounting,  which  is  usually  O  eye  size.  If 
the  fitting  glasses  are  too  narrow  in  P.  D.  increase  the  size  of 
the  lenses  until  the  proper  P.  D.  is  obtained,  provided  of 
course  that  it  is  not  more  than  a  few  millimeters  and  does  not 
make  the  lenses  too  large.  The  00  eye  lenses  are  one  milli- 
meter longer  than  0  eye  size  and  will  increase  the  P.  D.  just 
one  millimeter ;  000  eye  lenses  are  two  millimeters  longer  than 
0  eye  and  will  increase  the  P.  D.  the  same  amount.  You  do 
not  have  to  be  controlled,  however,  by  the  standard  sizes ;  000 
eye  lenses  have  a  length  of  41  mm.,  you  can  use  42,  43,  or  44 
mm.  lenses  if  you  desire.  There  is  usually  about  9  mm.  dif- 
ference between  the  length  and  breadth  of  regularly  shaped 
lenses,  so  you  can  specify  42  x  33  or  43  x  34,  etc.,  instead  of 


166  FRAME  FITTING 

trying  to  convert  these  lenses  to  a  standard  size.  Likewise 
where  it  is  desired  to  give  a  short  oval  effect  you  may  specify 
42  X  34  or  42  x  35,  etc.,  but  always  remember  that  when  you 
measure  the  P.  D.  of  a  pair  of  glasses  you  measure  from  the 
inside  edge  of  one  lens  to  the  outside  edge  of  the  other  lens 
and  in  this  way  the  length  of  only  one  lens  is  included  in  the 
total  P.  D.  and  consequently  an  increase  in  the  length  of  both 
lenses  of  2  mm.  will  increase  the  P.  D,  of  the  glasses  only  2 
mm,  and  not  4  mm.  as  might  at  first  be  supposed. 

Let  us  say  that,  in  order  to  cause  the  glasses  to  have  the 
proper  P.  D.  it  would  be  necessary  to  use  larger  lenses  than 
are  desired.  In  this  case  you  must  use  extended  posts ;  these 
correspond  to  the  C  and  D  studs  in  regular  eyeglass  mount- 
ings and  are  made  in  just  two  sizes,  1-16  and  1-8  inch.  Should 
you  put  on  1-16  extended  posts  you  will  increase  the  P.  D.  % 
inch  or  about  3  mm.  and  %  inch  extended  posts  would  increase 
the  P.  D.  %  inch  or  about  6  mm.  Here  it  will  be  seen  that 
both  posts  must  be  considered  in  the  P.  D.  as  we  include  them 
both  in  the  P.  D.  measurement. 

Now  observe  whether  the  lenses  are  too  close  or  too  far 
from  the  eyes,  if  so  prescribe  inset  or  outset  posts,  whichever 
set  and  inset  posts  are  made  in  two  sizes,  1-16  and  1-8  inch, 
and  it  will  be  found  comparatively  easy  to  judge  which  size 
are  needed,  the  same  as  when  fitting  regular  mountings.  Out- 
is  needed. 

Summing  up,  the  things  we  need  to  know  in  prescribing 
finger-piece  eyeglass  mountings  arc:  Ihc  number  or  size  of 
the  mounting,  extended,  inset  or  outset  posts  and  the  size  of 
the  lenses. 

In  fitting  glasses  that  contain  bifocal  lenses  care  must  be 
exercised  to  get  the  I'.  1).  exactly  right,  remembering  that  the 
P.  I).  f(jr  reading  is  usually  about  3  mm.  less  than  that  for 
flistant  \ision  .^o  that  the  reading  segments  must  be  moved 
nasalvvard.  'Jhe  lenses  also  should  set  a  trillc  lower  than 
single-focus  lenses  so  that  the  reading  segment  will  not  inter- 
fere w  ith  the  ])atient's  distant  \ision  and  the  lenses  should  also 
tilt  slightly  forward  so  the  stronger  or  reading  portion  of  the 
lenses  will  be  more  closely  at  right  angles  with  the  patient's 


FRANKLIN,  BENJAMIN  167 

line  of  vision  when  the  eyes  are  directed  downward  in  the 
natural  position  assumed  when  reading. 

Franklin,  Benjamin.  A  famous  American  statesman,  who  in- 
vented bifocal  glasses.    See  History  of  Optics. 

Franklin  Glasses.    Bifocal  glasses,  invented  by  Franklin. 

Function.  In  physiology  the  term  denotes  the  work  or  office  of 
each  separate  organ  (special  function)  or  of  the  body  as  a 
whole  (general  function).  In  mathematics  a  quantity  is  said 
to  be  the  function  of  another  quantity  when  a  modification 
of  the  former  involves  a  corresponding  modification  of  the 
latter. 

Fundus  Oculi.  All  that  part  of  the  eye  back  of  the  vitreous 
which  is  visible  through  an  ophthalmoscope,  including  the 
retina,  chorioid,  pigment,  vessels,  optic  disc,  etc.  (See  Oph- 
thalmoscopy) . 

Fuscin.  A  rich  brown  pigment.  Such  a  pigment  exists  in  the 
retinal  epithelium. 

Fusion.  In  physiologic  optics  this  word  signifies  the  faculty 
of  superimposing  the  two  central  images  of  the  retinae  and 
projecting  them  into  space  as  one  single  image. 

Next  to  the  desire  for  a  clear  image, — perhaps  equal  to  it, — 
is  the  imperative  desire  of  the  brain  for  a  single  image.  No 
doubt  it  would  be  more  correct  to  say  that  the  desire  for  a 
single  image  is  part  and  parcel  of  the  desire  for  a  clear  image, 
since  an  image  can  hardly  be  double  and  clear  at  the  same 
time.  Just  as  the  desire  for  clarity  will  impel  the  brain  to  use 
every  available  means  for  focussing,  up  to  the  last  ditch  of  its 
capacity,  so  will  the  desire  for  singleness  of  vision  lead  the 
brain  to  use  the  last  shred  of  its  converging  capacity. 

The  mechanics  of  fusion  will  be  found  discussed  under 
Convergence.  It  is  generally  supposed  that  there  is,  in  the 
frontal  lobe  of  the  brain,  a  fusion  centre  which  controls  this 
faculty.  The  whole  performance  is  one  of  co-ordination,  simi- 
lar to  standing  and  walking.  The  re-education  of  the  faculty 
is  the  prime  factor  in  the  cure  of  squint  or  imbalance,  just  as 


168 


GALEROPIA 


we  have  to  re-educate  the  co-ordination  of  the  locomotor 
ataxia  patient.  It  is  important  to  distinguish  between  this 
education  of  the  co-ordination  and  the  sheer  exercise  of  the 
muscles. 

Galeropia.    Galeropsia.    Abnormal  keenness  of  vision. 

Ganglion,  An  aggregation  of  nerve  cells  within  the  brain  or 
along  the  course  of  a  nerve.  Often  a  ganglion  serves  as  a 
point  of  junction  or  exchange  between  the  cerebro-spinal  and 
the  sympathetic  nerve  systems.  This  is  the  case  with  the 
important  ganglion  of  the  eye,  described  below. 

Ganglion. — Ciliary.  A  nerve  ganglion  situated  between  the 
optic  nerve  and  the  external  rectus  muscle  of  the  eye.  It  has 
three  roots:  (1)  A  long,  sensory  root,  from  the  nasociliary 
nerve,  (2)  a  short,  motor  root  from  the  oculomotor  (third 
nerve),  and  (3)  a  sympathetic  root.  It  gives  origin  to  the 
short  ciliary  nerves  which  go  to  the  coats  of  the  eyeball,  the 
ciliary  muscle,  and  the  iris.  Impulses  sent  out  along  the  third 
nerve  are  here  relayed  to  the  short  ciliary  nerves. 

Ganglion. — Gasserian.  A  ganglion  lying  in  the  fossa  on  the 
anterior  part  of  the  petrosa,  near  the  apex.  Its  chief  interest 
in  ophthalmology  is  its  remo\al  for  the  relief  of  trigeminal 
neuralgia  and  conse(|uent  keratitis. 

Generic  Compounds.  Compound  lenses  whose  curvatures  are 
developed  in  similar  ways,  i.  e.  whose  curvatures  are  of  the 
same  kind,  the  sphere  and  cylinder  both  convex  or  both  con- 
cave. When  the  curvatures  are  oi)posite  the  lenses  are  called 
contrageneric. 

Geometrical  Centre.  The  one  jxtiut  in  a  si)lK're  or  circle  which 
is  equidistant  from  all  points  in  the  circumference.  The  geo- 
metric centre  of  a  lens  or  mirror  is  the  geometric  centre  of  the 
sphere  of  which  it  is  a  segment. 

Geromorphism.  A  skin  disease  atTecling  the  upper  eyelid  and 
occasionally  i)ro(huing  ptosis. 

Glabella.  The  little  jjrcjininenci-  of  tlu-  skull  inuuiMhatrh-  bctwciu 
the    two    eyebrows.      bVom    the    glabilla    to    the    ociiput    (the 


GLAND  169 

prominence  at  the  back  of  the  head)  is  the  longitudinal  meas- 
ure of  the  skull. 

Gland.  An  organ  which  elaborates  a  secretion.  In  ophthalmol- 
ogy are  the 

Lachrymal  Glands,  which  secrete  the  tears. 
Sebaceous  Glands,  which  secrete  an  oily  substance  for  lub- 
ricating the  eyelids  and  eyeball,  and 

Meibomian  Glands,  which  are  really  larger  sebaceous  glands. 

Glass.  A  brittle,  transparent  substance,  composed  of  silica, 
potash  and  lead,  used  for  making  lenses.  The  index  of  refrac- 
tion depends  upon  the  relative  proportions  in  which  the  ingre- 
dients are  combined.  Crown  glass,  which  is  commonly 
employed  in  ordinary  lenses,  has  an  index  of  about  1.53.  Flint 
glass,  in  which  a  larger  proportion  of  lead  is  used,  has  an  index 
of  from  1.65  to  1.70.  The  former  has  a  higher  fusion  point 
than  the  latter,  which  permits  the  fusing  together  of  segments 
of  the  two  for  the  making  of  bifocal  lenses.     (See  Bifocal). 

Glaucoma.  A  serious  disease  of  the  eye  in  which  there  is  greatly 
increased  intraocular  tension,  the  cause  of  which  is  not  def- 
initely known,  but  supposed  to  be  an  impairment  of  the  func- 
tions of  the  canal  of  Schlem  and  the  spaces  of  Fontana  (q.  v.), 
so  that  the  anterior  chamber  is  not  adequately  drained.  The 
disease  may  be  acute  or  chronic;  primary  or  secondary,  i.  e. 
self-originating  or  following  some  inflammatory  disease.  The 
cardinal  symptoms  are : 

Severe  pain 

Tension  and  hardness  of  the  globe 

Failing  accommodation 

Dimness  of  vision 

Colored  halos  seen  around  a  light 

Cupping  of  the  optic  disc 

Steamy  cornea. 
The  use  of  atropine  in  the  eyes  of  persons  over  forty  is  apt 
to  produce  glaucoma,  because  it  crowds  the  iris  into  the  area 
of  the  spaces  of  Fontana,  and  increases  ocular  tension. 

Glaucomatous.     Pertaining  to  glaucoma. 


170  GLIOMA 

Glioma.  A  malignant  tumor  which  occasionally  invades  the 
retina. 

Glio-Sarcoma.     A  combination  of  glioma  with  sarcoma. 

Globulin.    A  protein  substance  contained  in  the  crystalline  lens. 

Goggles.  Large,  hollow  glasses  for  the  protection  of  the  eyes  in 
industrial  work,  automobiling,  etc.  For  industrial  purposes 
they  are  usually  furnished  with  wire  screens. 

Goitre.  Disease  of  the  thyroid  gland.  The  only  form  which 
interests  the  ophthalmologist  is  Exophthalmic  Goitre,  q.  v. 

Gonorrheal  Ophthalmia.  A  violent  inflammatory  disease  of  the 
eye  due  to  invasion  by  gonococci.  It  first  attacks  the  con- 
junctiva, causing  intense  redness,  swelling,  pain,  and  photo- 
phobia, followed  in  a  day  or  so  by  thick,  profuse,  yellow  dis- 
charge. If  not  quickly  checked,  it  attacks  the  cornea,  and  the 
interior  structures.  Indeed,  an  eye  is  frequently  destroyed  by 
the  disease  in  forty-eight  hours.  It  is  usually  unilateral,  ex- 
cept in  little  children,  who  carry  infection  to  the  eyes  with 
their  fingers  indiscriminately.  The  disease  is  highly  contag- 
ious, and  calls  for  prompt  isolation  and  treatment. 

Gould's  Sign.  Bowing  of  the  head  to  obtain  better  vision  in 
retinitis  pigmentosa. 

Graduated  Tenotomy.  An  incomplete  cutting  of  the  tendons  of 
the  rectus  muscles  to  effect  a  slight  amount  of  straightening 
of  the  eyes. 

Graefe's  Test.  A  test  for  heterophoria  by  means  of  a  prism,  base 
up  or  base  down,  before  the  eye,  to  double  the  image.  See 
Heterophoria. 

Granular  Lids.    See  Trachoma. 

Groove.  In  anatomy,  a  crease  or  furrow,  in  which  some  struc- 
ture usually  lies. 

Lachrymal    Groove.     The   grooxc    which    lodges    the   lach- 
rymal sac. 


HAAB'S  REFLEX  171 

Optic   Groove.     A  groove  on  the   superior   surface  of  the 
sphenoid  bone  in  which  the  optic  commissure  rests. 

Haab's  Reflex.  Contraction  of  the  pupil  when,  in  a  darkened 
room,  the  patient's  attention  is  directed  to  a  light  placed  to 
one  side  of  him,  the  eyes,  however,  not  being  turned  toward  it. 

Haidinger's  Brushes.  An  appearance  produced  when  polarized 
light  from  an  evenly  illuminated  surface  falls  on  the  eye.  See 
Polarization. 

Hair  Optometer.  An  English  instrument  for  measuring  the 
accommodation,  consisting  of  a  frame  like  a  miniature  harp, 
the  strings  being  replaced  by  small  hairs.  The  patient  holds 
this  against  a  white  background,  and  gradually  brings  it  nearer 
to  his  eye  until  he  can  distinguish  the  hairs.  The  distance 
from  the  instrument  to  the  eye  is  then  measured  by  a  steel 
tape,  graduated  into  centimeters  and  dioptric  values. 

Haller's  Circles.     Arterial  and  venous  circles  in  the  eye. 

Haller's  Tunic.    The  vascular  (second)  tunic  of  the  eye. 

Halo.  A  reddish-yellow  ring  of  color  surrounding  the  optic  disc, 
due  to  widening  of  the  scleral  ring. 

Halo  S5anptom.  The  sensation  of  colored  rings  around  a  light. 
It  is  a  frequent  symptom  of  glaucoma. 

Haplascope.  In  general,  any  instrument  for  measuring  the 
visual  axes.  Hering's  haploscope  is  an  apparatus  for  present- 
ing a  special  field  of  vision  to  each  eye,  while  the  two  are 
united  in  consciousness.  The  figures  consist  of  two  vertical 
lines,  separated  by  the  interpupillary  distance.  One  line  runs 
vertically  upward  from  a  point  representing  the  pupillary 
centre;  the  other  runs  vertically  downward  from  a  point  rep- 
resenting the  other  pupillary  centre.  When  each  eye  fixes  its 
own  central  point,  stereoscopically,  the  two  lines  are  seen  as 
one  continuous  line,  which,  however,  is  not  straight  and  ver- 
tical, but  bends  at  the  junction  of  the  two  lines  at  an  obtuse 
angle,  which  can  be  measured  angularly  by  means  of  the 
apparatus, 

Haplopia.    Single  vision,  as  distinguished  from  diplopia. 


172  HECTOMETER 

Hectometer.     One  hundred  meters. 

Helcosis.    Ulceration. 

Helmholz.  A  famous  physiologist  and  oculist.  See  History  of 
optics.     See,  also,  Accommodation,  Color. 

Hematoma,  Ocular.  Formation  of  blood  clots  in  the  vascular 
tissues  of  the  eye. 

Hemeralopia.     Day  blindness. 

Hemiablepsia.     See  Hemianopia. 

Hemiachromatopsia.     Color  blindness  in  half  of  each  retina. 

Hemiamblyopia.    Amblyopia  in  half  of  each  retina. 

Hemianopia.  Also  written  Hemianopsia.  Blindness  of  one  half 
of  each  retina.  When  the  two  corresponding  sides  are  blind, 
i.  e.  both  right  or  both  left  halves,  it  is  called  homonymous 
hemianopsia ;  when  the  non-corresponding  sides,  it  is  called 
heteronymous  hemianopsia.  In  the  latter  case,  we  designate  it 
as  either  nasal  or  temporal  hemianopsia.  It  is  usually  due  to 
grave  brain  lesions;  but  occasionally  to  hysteria. 

Hemierythropsia.    Red  vision  in  half  of  the  field. 

Hemophthalmia.     Extravasation  of  blood  into  the  eye. 

Hering's  Theory.    See  Color. 

Heterochromia.  A  mixture  of  color-pigments,  as  in  the  iris  or 
the  retina. 

Heterometropia.     Unequal  refractive  power  in  the  two  eyes. 

Heteronymous.  Applied  to  bilateral  alTections  which  are  un- 
symmetrical  or  non-ctMrespoiuling.  .Sec  Diplopia  am!  Hemian- 
opsia. 

Heterophoria.  In  the  section  on  Convergence  it  has  been  shown 
that  in  a  normal  |)air  of  eyes  the  following  conditions  obtain 
in  the  matter  of  the  ocular  muscles: 


HETEROPHORIA  173 

(1)  There  is  stable  equilibrium  between  the  various  pairs  of 
antagonistic  muscles  when  vision  is  directed  to  infinity ;  i.  e., 
the  far  point  of  convergence  is  at  infinity. 

(2)  The  amount  of  convergence  exerted  for  any  given  dis- 
tance within  infinity  is  equal,  in  prism  dioptres,  to  the  distance 
(in  meters)  divided  into  half  the  pupillary  width  (in  centi- 
meters). 

(3)  The  total  amount  of  convergence  achievable,  with  the 
accommodation  in  efifect,  is  approximately  30  to  32  prism 
dioptres;  i.  e.  the  near  point  of  convergence  is  approximately 
10  cm. 

Any  marked  departure  from  these  conditions  indicates  a 
defect  in  the  working  of  the  muscles,  and  constitutes  hetero- 
phoria,  or  muscle  imbalance. 

From  the  standpoint  of  resultant  effect,  i.  e.  the  position 
assumed  by  the  eye  in  a  state  of  stable  equilibrium,  hetero- 
phoria  is  classified  as  follows : 

Esophoria.     Turning  inward.  : 

Exophoria.     Turning  outward. 

Hyperphoria,  or  Anaphoria.     Turning  upward. 

Hypophoria,  or  Cataphoria.     Turning  downward. 

Cyclophoria.     Turning  obliquely. 

We  may  also  have  combinations  of  these  varieties,  hypere- 
sophoria,  hyperexophoria,  catesophoria,  catexophoria,  etc. 

From  the  point  of  view  of  the  underlying  cause,  we  distin- 
guish between  (a)  imbalance  due  to  anatomical  conditions,  and 
(b)  imbalance  due  to  errors  of  refraction. 

Strictly  speaking,  manifest  heterophoria  is  heterotropia. 
That  is  to  say,  where  the  patient  has  never  attempted,  or  has 
ceased  the  effort,  to  overcome  the  imbalance,  so  that  the  eyes 
take  up  a  constant  position  of  stable  equilibrium,  in  or  out  or 
up  or  down,  the  condition  ceases  to  be  one  of  muscular  imbal- 
ance and  becomes  a  squint,  which  will  be  treated  of  in  a  separ- 
ate section.  By  manifest  heterophoria.  however,  we  denote 
the  amount  of  imbalance  which  can  be  demonstrated  by  test; 
that  which  we  are  unable  to  bring  out  by  test  is  called  latent 
heterophoria. 


174  HETEROPHORIA 

DIFFERENTIATING  ANATOMICAL  FROM  FUNCTIONAL. 

There  is  no  doubt  that  imbalance,  as  well  as  strabismus,  is 
not  infrequently  due  to  structural  conditions  of  the  eye  mus- 
cles, both  anomalous  and  pathological ;  elongated  or  shortened 
muscles,  tendons  which  are  attached  too  far  forward  or  too  far 
back  on  the  eyeball,  and — most  frequent  of  all — partially  para- 
lyzed muscles.  And,  as  it  is  highly  important  that  the  refrac- 
tionist  should  know,  at  the  outset,  whether  he  is  dealing  with 
an  imbalance  due  to  this  kind  of  cause  or  to  one  that  pertains 
to  a  sheer  lack  of  coordination,  due  to  ametropia,  it  will  be  well 
to  point  out,  first,  the  ways  of  difYerentiating  between  them. 

The  elemental  principle  of  distinction,  upon  which  all  differ- 
entiating tests  are  based,  is  this :  an  anatomic  defect  will  show 
itself  in  every  movement  of  the  eye  in  which  the  affected 
muscle  is  involved,  whereas  a  purely  functional  imbalance  will 
be  manifested  only  when  the  muscle  or  muscles  are  engaged  in 
performing  the  perverted  function,  i.  e.,  convergence,  positive 
or  negative. 

The  simplest  and  most  obvious  test  is  to  ha\e  the  patient 
follow  with  his  eyes  an  object — the  operator's  finger,  a  pencil, 
etc. — as  it  is  moved  laterally,  vertically,  and  obliquely,  to  and 
fro,  without  changing  its  distance  from  the  eyes.  If  one  of  the 
muscles  of  either  eye  be  structurally  weak  or  insufficient,  the 
movement  of  that  eye  will  lag  as  the  movement  enters  the 
field  of  the  defective  muscle,  and  will  lag  more  and  more  the 
further  into  the  field  it  goes.  This  will  not  happen  in  a  sheer 
case  of  refractive  heterophoria,  for  eyes  that  simply  fail  to 
coordinate  in  convergence  on  account  of  ametropia  perform 
conjugate  movements  (|uite  normally. 

A  further  test  consists  in  covering  each  eye,  successively, 
while  the  patient  fixes  an  oI)ject,  ])referal)ly  at  infinity.  In 
both  forms  of  imbalance  the  co\  ered  eye  will  make  a  turn,  in 
or  out,  as  the  case  may  be,  because  tlie  need  for  fusion  is 
removed;  but  in  functional  heteroi»lioria  each  eye,  as  it  is 
covered  in  turn,  will  make  the  same  amount  of  de\iation,  while 
in  organic  heterophoria  the  sound  eye  will  make  decidedly  the 
greatest  turn  when  the  alTected  eye  is  fixing.  The  reason  for 
this  is  that  the  poor  eye  has  to  be  innervated  more  strongly 


HETEROPHORIA  175 

than  the  good  one  in  order  to  maintain  its  fixation,  and  this 
extra  innervation  is  imparted  also  to  the  covered  (good)  eye, 
causing  it  to  exaggerate  its  deviation. 

Duction  tests  will  furnish  confirmatory  evidence.  If,  under 
adduction  and  abduction  tests,  one  of  the  eyes  shows  a  marked 
departure  from  the  3  to  1  ratio  (See  Convergence)  between 
internal  and  external  muscle,  while  the  other  eye  shows  the 
normal  ratio,  it  is  pretty  strong  evidence  of  structural  trouble 
in  the  short-power  muscle. 

FUNCTIONAL  HETEROPHORIA. 

By  far  the  commonest  form  of  refractive  imbalance  is 
esophoria,  due  to  hyperopia.  It  is  probable,  indeed,  that  this 
is  the  only  form  which  can  be  attributed  directly  to  error  of 
refraction,  since  imbalance  comes  about  through  accommoda- 
tion-convergence disruption,  and  the  internals  are  the  only 
muscles  functionally  linked  with  accommodation.  In  the  sec- 
tion on  Convergence  it  is  shown  that  exophoria  is  a  normal, 
inevitable  accompaniment  of  the  act  of  convergence ;  that  most 
of  the  cases  of  refractive  exophoria  are  simply  exaggerated 
manifestations  of  this  accommodative  exophoria ;  and  that 
cases  of  exophoria  which  transcend  this  are  usually  due  to 
anatomic  causes. 

ESOPHORIA. 

There  is  no  such  thing,  however,  as  normal  or  accommoda- 
tive esophoria ;  for  there  is  no  normal  condition  calling  for 
compensatory  action  of  the  internals.  But  in  hyperopia,  where 
the  patient  has  to  accommodate  for  infinity,  there  is  an  instinc- 
tive tendency  on  the  part  of  the  internals  to  converge  in 
accordance  with  the  degree  of  accommodation  in  force.  This 
tendency  the  patient  resists,  out  of  his  desire  for  single  vision, 
and  there  is  thus  created  a  condition  of  unstable  equilibrium, 
or  imbalance,  tending  inward,  known  as  esophoria.  Esophoria 
is  thus  the  direct  result  of  hyperopia  and  exactly  proportionate 
to  it.  Thus,  if  a  patient  have  2  D.  of  hyperopia,  he  will  have 
2  meter  angles  of  esophoria,  or  about  6  prism  dioptres,  which 
will  be  the  same  at  any  distance,  although  it  is  usually  mani- 
fested most  markedly  at  infinity. 


176  HETEROPHORIA 

TESTS  FOR  ESOPHORIA. 

The  presence  of  esophoria  can  be  roughly  determined  by 
Duane's  test,  or  the  cover  test,  already  referred  to,  namely,  by 
having  the  patient  fix  an  object  at  infinity  with  both  eyes, 
suddenly  covering  one  of  his  eyes  with  a  card,  in  such  a  way, 
however,  that  the  operator  can  still  observe  the  covered  eye. 
If  esophoria  is  present,  the  covered  eye  will  make  a  turn 
inward. 

Quantitative  estimation  of  esophoria  must  be  made  by 
means  of  dissociation  tests,  a  full  description  of  which  will  be 
found  in  the  section  on  Convergence.  Neither  the  Maddox 
rod  nor  the  dot  and  line  test  will,  as  a  rule,  disclose  the  full 
amount  of  esophoria  present,  because,  in  spite  of  dissociation, 
the  mental  faculties  of  the  patient  still  induce  a  certain  amount 
of  attempt  to  fuse  the  images.  The  patient  knows  that  he  is 
expected  to  fuse  them,  and  tries  to  do  so.  This  amount  of 
latent  imbalance,  however,  need  give  us  no  concern ;  for  what- 
ever the  patient  is  able  to  overcome  under  these  conditions 
will  certainly  give  him  no  trouble  in  his  ordinary  use  of  the 
eyes. 

EXOPHORIA. 

As  previously  stated,  most  cases  of  so-called  exophoria  at 
near  point,  in  myopes,  are  but  exaggerated  manifestations  of 
accommodative  exophoria.  The  myope  uses  little  or  no 
accommodation  at  near  point ;  most  of  his  convergence  is 
stimulated  by  his  fusion  faculty,  and  when  the  fusion  sense  is 
destroyed  by  dissociating  the  images  it  is  readily  released. 
This  condition,  in  young  adults  who  have  not  yet  become 
presbyopic,  rarely  gives  any  trouble  or  calls  for  any  aid. 

In  presbyopes  whose  accommodation  has  fallen  to.  or  near, 
zero  the  same  state  of  affairs  obtains ;  in  them  it  frequently 
proves  troublesome  merely  because  their  fusion  faculty  easily 
tires. 

Genuine  exophoria  is  practically  always  due  to  mechanical 
or  anatomic  causes,  chief  among  which  is  the  difficulty  of 
rotating  the  elongated  eyeball.  This  type  of  exophoria  mani- 
fests itself  at  infinity  as  well  as  at  near  point,  and,  in  most 
cases,   ()ui(.kl\-   dc\clops   into   cxotropia.      Indeed,   it   may   be 


HETEROPHORIA  177 

remarked   that    the    great    majority   of    apparent    exophorias, 
manifested  at  infinity,  are  really  slight  exotropias. 

TESTING  FOR  EXOPHORIA. 

Exophoria  is  tested  and  estimated  in  the  same  way  as 
esophoria,  namely,  by  dissociation  of  the  images  by  means  of 
Maddox  rod,  dot  and  line,  and  index  pointer,  and  by  prism 
measurement. 

HYPERPHORIA  AND  CATAPHORIA. 

There  is  nothing  either  in  hyperopia  or  in  myopia  to  induce 
vertical  imbalance ;  it  may  therefore  be  safely  considered  that 
all  cases  of  hyperphoria  and  cataphoria  are  due  to  mechanical 
or  anatomical  causes.  A  misplaced  macula  is  sometimes  the 
cause. 

The  tests  for  vertical  imbalance  are  the  same  as  for  lateral 
imbalance,  except  that  the  Maddox  rod  and  the  chart  are  to  be 
placed  at  right  angles  to  the  position  given  them  in  testing 
lateral  cases ;  in  this  way  the  bar  of  light  made  by  the  Maddox 
rod  will  lie  horizontally  across  the  visual  field,  showing  the 
displaced  image  above  or  below  it,  and  the  apparent  displace- 
ment of  the  dot  or  the  index  pointer  will  be  up  or  down  the 
graded  line. 

Vertical  imbalance  is  spoken  of  as  being  right  or  left, 
according  as  it  is  the  right  or  left  eye  which  deviates  upward 
or  downward  under  the  test.  However,  it  can  be  quantitatively 
estimated  by  a  prism  in  front  of  either  eye,  as  the  superior  and 
inferior  rectus  of  the  two  eyes  counteract  each  other  in  im- 
balance. 

CYCLOPHORIA. 

Occasionally  there  occur  cases  of  heterophoria  in  which  the 
oblique  muscles  play  a  decided  role,  so  that  there  is  a  tendency 
to  rotary  imbalance.  This  condition  is  known  as  cyclophoria, 
and  manifests  itself  under  test  by  an  oblique  position  of  the 
objects  viewed.  It  is  corrected  by  a  resultant  prism,  with  the 
base  in  the  direction  of  the  angular  displacement  of  the  image. 

An  excellent  test  for  cyclophoria  is  the  old  cone  test.  A 
cone  of  glass,  cemented  onto  a  ground  glass  disk,  is  mounted 
and  centered  before  one  eye,  the  other  being  meanwhile  cov- 
ered with  an  opaque  disk.     Attention  is  directed  to  a  candle 


178  HETEROPHORIA 

flame.  The  cone  draws  the  flame  out  into  a  circle  of  light. 
The  other  eye  is  now  uncovered.  If  there  be  no  imbalance,  the 
candle  flame  will  be  seen  in  the  centre  of  the  circle  of  light. 
If  there  be  cyclophoria,  the  flame  will  be  displaced  outside  the 
circle,  and  the  direction  in  which  it  is  displaced  will  indicate 
where  the  base  of  the  correcting  prism  must  be  placed. 

TREATMENT  OF  HETEROPHORIA 
There  is  no  more  disputed  subject  in  the  realm  of  refraction 
than  the  treatment  of  hcterophoria.  One  can  lay  down  only  a 
few  guiding  rules,  or  principles,  to  govern  the  management  of 
such  cases,  leaving  the  details  of  treatment  to  be  worked  out 
in  each  individual  case. 

ESOPHORIA. 

Since  esophoria  is  almost  always  the  direct  result  of  hyper- 
opia, it  is  usually  only  necessary  to  correct  the  hyperopia  to 
remedy  the  imbalance,  especially  if  the  degree  of  imbalance 
does  not  exceed  that  which  is  proper  to  the  amount  of  hyper- 
opia present,  i.  e.  in  prism  dioptres,  three  times  the  dioptres  of 
hyperopia.  If  it  be  greatly  in  excess  of  this  proportion,  or  if 
it  give  much  trouble,  still  an  attempt  ought  to  be  made  to 
remedy  the  trouble  without  resort  to  prism  correction,  giving 
the  lens  correction  plenty  of  time  to  assert  its  influence,  and 
perhaps  giving  the  patient  exercises  in  fusion.  If  prism  cor- 
rection seems  imperative,  then  a  careful  examination  should 
be  made  to  determine  the  least  possible  amount  of  prism  cor- 
rection that  will  enable  the  patient  to  maintain  fusion,  and  that 
amount  barely  prescribed,  so  as  to  keep  the  fusion  faculty  in 
exercise,  reducing  the  prism  from  time  to  time  as  the  patient 
can  bear  the  reduction. 

EXOPHORIA. 

Exophoria  is  not  so  easy  to  dogmati/.c  about,  because  it  has 
not  the  same  direct  relation  to  myojiia  that  csojihoria  has  t«) 
hyperopia.  The  rule  is  to  gi\e  only  two-thirds  or  three-fourths 
of  whatever  exophoria  there  is  at  near  ])()int  in  excess  of 
accommodati\  e  exophoria.  The  strong  probability  is,  how- 
ever, that  if  it  can  once  be  established  that  exophoria  is  present 
other  than  accommodative  exophoria,  it  will  eventually  have 
to  be  corrected  by  prisms,  and  this  might  as  well  be  done  at 


HEtEROPHORlALGIA  179 

once.      Certainly,    exophoria    strongly    manifested    at   infinity 
usually  calls  for  full  prism  correction. 

HYPERPHORIA  AND  CATAPHORIA. 

As  the  movements  of  these  muscles  are  in  reality  conjugate 
movements,  it  is  highly  probable  that  all  cases  of  vertical 
imbalance  are  due  to  anatomic  conditions,  and  that  prism  cor- 
rection will  be  found  necessary  in  every  instance. 

MUSCLE  EXERCISES. 

As  to  the  value  of  muscle  exercises,  by  means  of  prisms  and 
other  methods,  in  cases  of  heterophoria,  there  is  a  wide  latitude 
of  opinion.  Certainly  such  exercises  must  be  carried  out  with 
great  intelligence  and  caution,  for  fear  of  doing  more  harm 
than  good.  This  subject  will  be  found  discussed  under  the 
heading  of  Muscle  Exercises. 

Heterophorialgia.     Pain  due  to  muscular  imbalance. 

Heterophthalmus.  Usually  caused  by  heterochromia  of  either 
or  both  irides,  making  the  two  eyes  look  different. 

Heterotropia.  When  a  pair  of  eyes  in  which,  for  any  reason, 
muscular  imbalance  exists,  no  longer  maintain  parallelism  or 
fusion,  but  take  up  a  more  or  less  permanent  position  of 
stable  equilibrium  with  their  visual  axes  either  convergent  or 
divergent,  this  condition  of  manifest  heterophoria  is  known  as 
heterotropia.  It  is  also  commonly  known  as  strabismus,  or 
squint.  For  a  detailed  discussion  of  the  subject,  see  Strabis- 
mus. 

History  of  Optics.  The  history  of  optics,  like  that  of  every 
physical  science,  is  a  widely  distributed  affair,  and  reaches 
into  remote  ages  and  into  every  part  of  the  world.  Thinkers 
and  workers,  of  every  period,  and  every  race,  have  contributed 
to  its  evolution  and  upbuilding,  from  Aristotle  down,  and  it 
would  be  impossible,  in  a  work  of  this  limited  scope,  to  give 
even  an  abridged  account  of  its  history  from  the  beginning. 

The  modern  history  of  optics,  at  least  so  far  as  the  refrac- 
tionist  is  concerned,  may  be  said  to  ha\e  its  beginning  with 
the  discovery  by  Willebrord  Snell,  a  Dutch  astronomer  and 
mathematician,  of  the  law  of  refraction.     Snell  was  born  at 


180  HISTORY  OF  OPTICS 

Leiden,  in  1591,  and  succeeded  his  father  as  professor  of 
mathematics  in  the  University  of  Leiden.  It  was  in  1621  that 
he  discovered  the  law  of  refraction,  that  "the  ratio  of  the  sines 
of  the  angle  of  incidence  and  the  angle  of  refraction  is  con- 
stant." His  discovery,  however,  was  not  published  until  ten 
years  after  his  death,  when  it  was  announced  in  a  work  by 
Descartes,  without  any  reference  to  Snell's  name,  in  1635. 
Whether  Descartes  had  access  to  Snell's  manuscript,  or  made 
the  discovery  on  his  own  part,  is  a  disputed  question. 

Snell's  original  manuscript  did  not  set  forth  the  law  in  the 
form  in  which  we  now  state  it.  It  expressed  it  as  the  ratio  of 
certain  lines  trigonometrically  interpretable  as  sines  of  cose- 
cants. Descartes  expressed  the  law  in  its  modern  trigonomet- 
rical form,  i.  e.  as  the  ratio  of  the  sines  of  the  angles  of  inci- 
dence and  refraction. 

In  his  work  on  the  system  of  nature,  Descartes  developed  a 
theory  of  light  which  in  some  respects  resembled  the  old 
Aristotelian  theory  and  in  some  respects  approximated  the 
modern  wave  theory.  He  regarded  light  as  a  pressure,  trans- 
mitted by  an  infinitely  elastic  medium,  and  color  as  being  due 
to  rotary  movements  of  the  particles  of  this  medium.  He 
attempted  a  mechanical  explanation  of  refraction,  and  held 
that  the  more  highly  refractive  the  medium  the  more  readily 
light  passed  through  it.  His  views  were  opposed  by  Pierre  de 
Fermat,  who  argued  that,  since  Nature  performs  her  opera- 
tions by  the  shortest  possible  paths,  the  path  traversed  by  a 
ray  of  light  between  two  points  must  be  such  that  the  time 
occupied  is  a  minimum.  This  law  he  termed  the  "law  of  least 
time."  He  pointed  out  that  the  linear  propagation  of  light 
and  the  law  of  reflection  agreed  with  this  principle,  and  that 
the  law  of  refraction  would  probably  be  found  to  correspond 
with  it.  While  his  premises  were  wrong,  his  conclusions  were 
right,  and  led  to  Sir  William  Hamilton's  concci)tion  of  "char- 
acteristic function"  by  which  all  ()])tical  problems  are  solved. 

Up  to  1()76  the  velocity  of  li^ht  propagation  had  been 
regarded  as  infinite.  In  that  year,  Ole  Roemer.  a  Danish 
astronomer,  born  in  \(A4,  and  laltT  Astronomer  Royal  to  tlie 
Copenhagen  observatory,  dediuiMl  tlu-  llnite  \i-K»(.ity  of  light 
frcjui   a  roiiiparison  betwi'cn   obsiTv cd  and   coinpiiti'tl   tiiiu'S  i)f 


HISTORY  OF  OPTICS  181 

the  eclipse  of  the  moons  of  Jupiter.     This  velocity  he  calcu- 
lated as  186,C)(X)  miles  per  second,  in  luminous  ether. 

In  1678  Christiaan  Huygens,  a  Dutch  physicist,  enunciated 
his  famous  theory  of  the  undulatory  nature  of  light,  upon 
which  all  of  our  modern  science  and  art  of  refraction  are 
based  (see  Light).  For  many  years  this  theory  of  light  was 
overshadowed  by  the  corpuscular  theory  of  Newton,  owing  to 
the  tremendous  authority  of  the  English  mathematician.  Not 
until  the  early  part  of  the  nineteenth  century  did  the  corpus- 
cular theory  meet  any  serious  opposition.  Its  chief  opponents 
were  Thomas  Young,  Fresnel,  Neumann,  Green  and  Stokes; 
but  it  was  J.  B.  L.  Foucault  who  finally  overthrew  the  Newton- 
ian doctrine  and  established  Huygens'  wave  theory  by  showing 
that  the  velocity  of  light  was  less  in  water  than  in  air. 

Meantime,  the  evolution  of  the  scientific  and  philosophic 
aspects  of  optics  was  accompanied  by  a  corresponding  prog- 
ress in  the  material  side  of  the  subject,  i.  e.  in  the  invention 
and  manufacture  of  optical  instruments.  The  grinding  of 
spherical  lens  surfaces  was  greatly  improved  by  Huygens,  who 
also  did  considerable  work,  but  without  much  result,  upon 
the  production  of  an  achromatic  lens.  Not  until  1757  was  this 
object  achieved  by  John  Dollond,  an  English  optician,  born 
in  1706,  originally  a  silk  weaver  of  Spitalfields,  London.  He 
first  made  an  achromatic  lens  by  a  combination  of  glass  and 
water  media,  and  then  succeeded  in  accomplishing  the  same 
end  by  combining  difterent  densities  and  qualities  of  glass. 

In  1784,  Benjamin  Franklin,  on  this  side  of  the  Atlantic, 
invented  bifocal  lenses ;  and  in  1837  Schnaitman  took  out  a 
patent  on  one-piece  lenses,  in  which  the  distance  and  the  read- 
ing portions  of  the  lens  were  ground  upon  one  piece  of  glass. 

In  1844  the  theory  and  mechanics  of  lenses  experienced  an 
epoch-making  impetus  in  the  work  of  Karl  Freiderich  Gauss, 
the  distinguished  German  astronomer  and  mathematician,  who 
was  born  in  1777.  Gauss'  work,  in  determining  focal  points 
and  image  representations,  was  in  later  years  supplemented 
by  that  of  E.  Abbe,  and  by  Listing,  who  in  1845  demonstrated 
nodal  points. 

In  1855  the  ophthalmoscope  was  invented  by  Hermann  Lud- 
wig    Ferdinand    von    Helmholz,    the    German    physicist    and 


182  HISTORY  OF  OPTICS 

physiologist,  who  was  born  in  Potsdam  on  the  31st  of  August 
1821.  In  1871,  Cuignet  invented  the  retinoscope,  which,  of 
course,  is  an  appHcation  of  the  principle  and  method  of  the 
ophthalmoscope.  Helmholz  also  devised  the  principle  of  the 
ophthalmometer,  for  measuring  the  curvature  of  the  cornea ; 
and  he  constructed  an  ophthalmometer,  but  for  many  reasons 
it  was  not  a  practical  success.  The  first  practicable  instru- 
ment was  built  in  1882  by  Javal,  which  constitutes  the  model 
of  our  modern  instruments. 

One  of  the  latest  achievements  in  technical  optics  is  the 
invention  and  construction  of  lenses  of  such  composition  as 
to  prevent  the  passage  of  the  ultra-violet  and  other  rays.  This 
was  accomplished  in  1912  by  Sir  William  Crookes,  the  English 
physicist  who  devised  the  first  tube  for  developing  the  X-ray, 
and  the  lenses  are  called  by  his  name. 

In  the  development  of  the  clinical  side  of  optics — physio- 
logic optics,  three  names  stand  out  so  far  above  all  the  rest 
that  they  may  be  mentioned  without  any  fear  of  rivalry, — 
Helmholz,  Young  and  Bonders. 

Helmholz  has  already  been  mentioned  as  the  inventor  of 
the  ophthalmoscojje.  He  is  also  responsible  for  invaluable 
work  in  the  physiologic  field  of  optics,  notably  for  a  demon- 
stration of  the  function  of  accommodation,  and  the  formula- 
tion of  a  theory  of  its  mechanism  which  dominates  the  field  of 
teaching  to  the  present  day,  and  the  joint  enunciation  with 
Young  of  a  theory  of  color-jierception,  which  will  be  found 
discussed  in  its  proper  section. 

Thomas  Young  was  an  English  physicist,  born  at  Milverton. 
Somersetshire,  of  a  Quaker  family,  in  1773.  He  anticipated 
Helmholz  by  several  years  in  a  general  explanation  of  the 
mode  by  which  the  eye  accommodates  itself  to  vision  at  dif- 
ferent distances  by  changing  the  curvature  of  the  crystalline 
lens.  He  originated  the  theory  of  color  perception  which 
Helmholz  afterwards  develoijcd.  He  was,  moreover,  the  dis- 
coverer of  the  phenomenon  of  the  interference  of  light  waves. 

Frans  Cornelius  Donders  was  a  Dutch  physician,  who  was 
born  in  1818  at  Tilburg,  Hollaiul,  and  aftcrwar«l  became  IMo- 
fessor  of  Physi(jlogy,  ilistoloj^y  and  ( )plitlialnu)logy  in  the 
University  oi  Utrecht,  wlure  he  hiiuself   had   lieen  educated. 


HIPPUS  183 

He  was  contemporary  with  Helmholz.  He  did  a  tremendous 
-amount  of  important  work  on  ocular  refraction,  especially  in 
regard  to  the  physiology  of  accommodation  and  presbyopia, 
and  enunciated  some  laws  governing  these  phenomena  (See 
Laws).  To  Bonders,  in  fact,  we  owe  most  of  our  modern 
knowledge  of  the  subject,  and  he  may  well  be  called  the  father 
of  modern  refraction. 

In  the  last  fifty  years,  and  especially  in  the  last  twenty-five 
years,  optics  has  made  enormous  strides,  both  as  a  science  and 
also  as  a  profession,  and  the  number  of  able  men  that  have 
contributed  to  its  progress  is  legion.  However,  in  this  more 
recent  period  very  few,  if  any,  really  basic  principles  have 
been  discovered,  or  fundamental  innovations  made. 

Hippus.    Iridodonesis ;  spasmodic  movements  of  the  iris. 

Hippus,  Respiratory.  Dilation  and  contraction  of  the  pupils,  in 
rythm  with  inspiration  and  expiration,  respectively. 

Hirschberg's  Method.  Using  reflection  of  a  candle  from  the 
cornea  to  measure  the  amount  of  deviation  of  a  strabismic  eye. 

Holmgren's  Test.  A  test  for  color  blindness.  See  Color  Blind- 
ness. 

Holocain.  A  local  anesthetic  drug,  often  used  in  the  eye,  either 
in  place  of,  or  in  combination  with,  cocaine.  It  is  four  times 
as  anesthetic  as  cocaine,  non-toxic,  does  not  afifect  the  cornea 
(as  cocaine  does),  and  is  soluble  in  40  parts  of  water. 

Homatropine.  A  mydriatic  agent,  much  used  in  the  eye  where 
it  is  desired  to  obtain  a  temporary  dilatation  of  the  pupil  or 
suspension  of  accommodation  for  purposes  of  examination. 
Its  efifects  wear  off  in  about  18  to  24  hours.  It  is  commonly 
used  in  a  2  per  cent  solution,  a  drop  being  put  into  the  eye 
every  four  or  five  minutes  for  an  hour.  Good  dilatation  is  then 
present. 

Homocentric.  Seeking  a  common  centre.  As  applied  to  light 
this  implies  that  the  rays  have  been  rendered  convergent  by  a 
mirror  or  a  lens  that  is  a  segment  of  a  sphere,  so  that  they  are 
converging,  in  the  form  of  a  cone,  to  one  focal  point. 


184  HOMOGENEOUS 

Homogeneous.  A  body  is  said  to  be  homogeneous  whose  sub- 
stance is  of  the  same  physical  character  and  properties  through- 
out. Such  a  body,  of  course,  has  a  uniform  effect  upon  any 
form  of  energy  (as  Hght)  which  passes  through  it  as  a  medium. 
Optical  glass  is  homogeneous. 

Homogeneous  Immersion.  A  method  in  microscopy  in 
which  the  object  to  be  viewed  and  the  objective  lens  are  both 
immersed  in  a  continuity  of  oil  of  the  same  optical  density  as 
the  lens,  thus  increasing  the  magnifying  power  of  the  latter. 

Homonymous.  Like,  or  corresponding.  Applied  especially  to 
diplopia  in  which  the  images  separate  in  the  same  direction  as 
the  eyes  that  see  them,  and  to  half-blindness  of  corresponding 
halves  of  the  retinae. 

Hordeolum.  Stye.  Infection  of  hair  follicle  of  the  lid.  Suc- 
cessive crops  of  styes  are  often  due  to  eyestrain  from  errors 
of  refraction. 

Horny  Epithelium.  Hard,  hypertrophied  state  of  the  epithelium, 
such  as  is  often  seen  in  the  conjunctiva  in  trachoma. 

Horopter.  The  area  included  in  the  field  of  binocular  vision. 
See  Binocular  Vision. 

Hot  Eye.  A  temporary  congestion  of  the  eye,  frequently  met 
with  in  gouty  subjects. 

Humor,  I'luid  content  of  the  chambers  of  the  eye.  The  ai|ueous 
humor  fills  the  anterior  chamber;  the  \itrcous  humor  the 
portion  of  the  eye  back  of  the  lens. 

Hutchinson's  Pupil.     I'nihitcral  (lilation  of  the  pupil. 
Hyalitis.     Inflammation  of  the  vitreous. 

Hyaloid.  Litirally,  resembling  glass.  It  is  applied  to  anything 
which  pertains  to  the  vitreous. 

Hyaloid  artery.  Tiranch  of  the  central  retinal  artcr\-  which 
in  the  fetus  jiierccs  the  vitreous  for  its  nourishment. 

Hyaloid  canal.  The  canal  through  which  hyaloid  artery 
passes.  The  canal  persists  in  extra-uterine  life.  It  is  also 
called  the  Canal  of  Stilling. 


HYDROPHTHALMIA  185 

Hyaloid  Fossa.  The  depression  in  the  anterior  surface  of 
the  hyaloid  membrane  into  which  the  crystalline  lens  is  set. 

Hyaloid  Membrane.  The  membrane  surrounding  the  vitre- 
ous, and  forming  the  suspensory  ligament  and  zonula. 

Hydrophthalmia.     Increase  in  watery  fluids  of  the  eye. 

Hygiene  of  the  Eye.  So  far  as  the  visual  function  is  concerned, 
the  hygiene  of  the  eye  falls  into  two  general  divisions,  namely, 
(1)  that  which  pertains  to  the  muscle  elements,  accommoda- 
tion and  convergence,  and  (2)  that  which  has  to  do  with  the 
retina,  to  which  may  be  added  (3)  that  which  concerns  the 
ordinary  care  of  the  eyes  themselves. 

(1)     MUSCULAR  HYGIENE. 

Close  Application.  Where  continued  contraction  of  a 
muscle  or  a  group  of  muscles  is  maintained  for  long  periods 
in  carrying  out  work  upon  which  the  attention  is  fixed,  the 
sense  of  fatigue  does  not  manifest  itself  until  a  rest  is  taken, 
or  until  the  muscles  give  out  and  refuse  to  contract  longer. 
This  is  precisely  what  takes  place  in  the  ciliary  and  internal 
rectus  muscles  of  the  eye  when  the  vision  is  concentrated  at 
near  points  for  any  considerable  length  of  time.  In  addition, 
the  continued  and  exces:ive  demand  of  the  muscles  for  blood 
during  their  activity  induces  congestive  troubles  in  the  eye- 
balls and  eyelids. 

Frequent  Rests.  For  these  reasons  the  vision  should  not  be 
exercised  continuously  at  near  point.  Persons  whose  occupa- 
tion obliges  them  to  work  at  close  range  should  make  a  practice 
of  giving  their  eyes  frequent  short  rests  by  removing  them 
from  their  work  and  relaxing  their  accommodation  and  con- 
vergence for  a  few  minutes  by  looking  into  the  distance. 

Children's  Eyes.  The  mischievous  results  of  continued 
maintenance  of  close  vision  are  particularly  marked  in  young 
children,  whose  musculature  is  in  the  formative  stage.  For- 
tunately we  are  nowadays  waking  up  to  the  dangers  of  this 
abuse,  and  are  taking  steps  to  avoid  them. 

Children  should  not  be  required  to  decipher  very  small  or 
close  characters  at  all.  Their  books  should  all  be  printed  in 
moderately  large  and  very  plain  type,  and  held  at  a  proper 


186  HYGIENE  OF  THE  EYE 

distance  from  the  eye.  And,  in  the  second  place,  their  tasks 
should  be  arranged  so  as  to  give  them  the  least  possible 
amount  of  close  work,  and  the  greatest  possible  alternation  of 
work  requiring  no  visual  effort. 

All  of  the  above-mentioned  evils  of  close  application,  both  in 
children  and  in  adults,  are  of  course  intensified  a  hundred-fold 
when  any  error  of  refraction  exists. 

(2)     RETINAL  HYGIENE. 

Poor  Illumination.  Near  work  done  under  poor  illumina- 
tion has  precisely  the  same  ill  effect  as  that  which  is  done 
under  conditions  of  excessive  exactingness.  The  inadequate 
intensity  of  the  image  on  the  retina  requires  that  the  image 
be  held  there  for  a  greater  length  of  time  in  order  to  make  the 
proper  impression  on  the  brain.  Thus  the  musculature  and  the 
central  nervous  system  both  suffer.  The  question  of  what 
constitutes  inadequate  illumination  will  be  found  discussed  in 
the  section  on  Illumination. 

Excessive  Illumination.  Too  much  light  is  almost,  if  not 
quite,  as  bad  as  too  little.  A  moderate  degree  of  irritation  of 
the  retinal  nerves  by  light  is  necessary  to  the  production  of 
sensation  of  vision;  but  if  the  stimulation  be  too  vigorous,  the 
nerve-ends  of  the  retina  quickly  become  inflamed  or  exhausted, 
and  retinitis  or  retinal  asthenopia  ensues.  Besides,  bright  light 
is  rich  in  ultra-violet  actinic  rays,  which  are  harmful  to  the 
retina.  Facing  continuously  a  bright  window,  or  doing  one's 
work  by  a  hard,  brilliant  light  should  be  avoided;  and  if  one's 
employment  makes  such  practices  unavoidable,  tlicn  he  sliould 
wear  tinted  glasses  to  protect  the  eyes. 

Glittering  Materials  and  Colors.  A  frequent  cause  of  this 
type  of  eye-strain  is  the  excessive  rellection  and  scintillation 
of  light  by  the  object  upon  which  one  is  working.  Printers 
and  metal  workers  experience  this  kiml  of  trouble.  The  only 
efficient  j)ro]jhyla.\is  is  to  wear  tinted  glasses,  and  to  take  fre- 
quent short  periods  of  rest. 

Those  who  are  obliged  to  concentrate  their  eyes  upon  one  or 
more  bright  colors  suffer  not  only  from  general  retinal  exhaus- 
tion, but  from  a  specific  exhaustion  of  these  elements  which 
respond  to  the  colors  in  question.    Their  only  means  of  relief 


HYGIENE  OF  THE  EYE  187 

is  the  wearing  of  tinted  glasses  of  such  a  color  as  to  neutralize 
the  offending  color  into  a  more  or  less  mixed  and  quiet  tint. 
Failing  this,  they  must  rest  the  fatigued  retina  by  gazing  fre- 
quently at  the  color  which  is  complementary  to  the  one  which 
distresses  them. 

(3)     CARE  OF  THE  EYE  ITSELF. 

Dust.  A  common  source  of  eye  trouble,  especially  in  large 
cities,  and  in  these  days  of  the  automobile,  is  in  the  dust  that 
enters  the  eye  and  irritates  the  conjunctiva.  It  is  a  frequent 
cause  of  conjunctivitis  and  pterygium.  The  only  effective 
safeguard  is  the  wearing  of  goggles,  to  which  the  ordinary 
man  or  woman  can  hardly  be  expected  to  submit.  Those  who 
are  in  the  dust  a  great  deal  should  wash  their  eyes  every  even- 
ing with  a  copious  irrigation  of  boric  acid  solution. 

Cold.  The  membrane  of  the  eye  is  subject  to  the  same  ill 
results  of  sudden  and  extreme  changes  of  temperature  as  other 
membranes  of  the  body.  It  is  able  to  adjust  itself  to  moderate 
variations,  but  violent  changes,  especially  when  accompanied 
by  high  winds,  will  congest  the  conjunctiva  and  induce  con- 
junctivitis. When  facing  an  extraordinarily  severe  cold  or 
heat  the  lids  should  be  kept  closed  for  a  few  minutes  until  the 
conjunctiva  becomes  gradually  accustomed  to  the  change.  In 
cases  of  very  extreme  exposures,  of  course,  goggles  should  be 
worn. 

Infection.  The  conjunctiva  has  a  very  ready  and  rapid 
absorptive  capacity,  and  presents  a  most  facile  field  for  inva- 
sion by  infectious  material  of  all  kinds.  Scrupulous  care  should 
be  exercised  at  all  times  to  avoid  this  kind  of  injury.  When- 
ever it  is  necessary  to  manipulate  the  eye  or  eyelids  the 
hands  should  be  carefully  washed ;  if  a  handkerchief  or  cloth 
is  used,  it  should  be  scrupulously  clean ;  any  drops  or  wash  for 
the  eyes  should  be  prepared  with  sterile  water;  and  after  any 
such  manipulation  the  eye  should  be  thoroughly  flushed  with 
warm  boric  acid  or  other  mild  disinfectant  solution. 

Such  precautions  are  especially  important  in  the  case  of 
persons  suffering  from  an  infectious  disease  in  other  parts  of 
the  body,  especially  if  accompanied  by  a  discharge.  If  habits 
of  carefulness  be  formed  during  times  when  there  is  no  emer- 


188  HYOSCINE 

gency,  they  will  be  exercised  automatically  when  emergencies 
exist. 

Posture.  Posture  has  a  powerful  influence  upon  the  health 
and  development  of  the  eye.  Poring  over  a  book  with  the  head 
bent  almost  to  the  knees,  so  common  among  children,  induces 
general  congestion  of  the  eyeball,  overproduction  of  secretions, 
stretching  of  the  choroid,  bringing  about  choroiditis  and  pro- 
gressive myopia  by  elongation  of  the  eyeball.  Myopes  are 
particularly  guilty  of  this  habit. 

All  persons,  and  especially  myopes,  in  reading  and  close 
work,  should  form  the  habit  of  sitting  upright  and  holding 
their  work  at  a  comfortable  distance  from  the  eyes.  If  their 
refraction  will  not  permit  them  to  do  this  it  should  be  corrected 
so  as  to  enable  them  to  do  it. 

Hyoscine.  An  alkaloid  of  hyoscyamus,  sometimes  used  as  a 
mydriatic. 

Hyperemia.  Congestion  of  the  blood  vessels.  This  condition 
is  seen  in  all  infections  and  inflammations  of  the  eye. 

Hyperesophoria.  Condition  arising  from  muscular  insufticiency 
causing  one  eye  to  deviate  upward  and  inward. 

Hyperesthesia.     Exaggerated  reaction  to  stimulus. 

Retinal  hyperesthesia.     Excessive  sensitiveness  to  light. 

Hyperexophoria.  Condition  arisinj;  from  muscular  insuflicicncy 
causing  one  eye  to  deviate  ui)war(l  and  outward. 

Hyperkeratosis.    1  lypcrtro])hy  of  the  corneal  tissue. 

Hypermetrcpia.     Sec  Hyperopia. 
Hyperope.     .A  ]>('rson  who  has  hyperopia. 

Hyp>€ropia.  That  condition  of  refraction  in  which  the  poste- 
rior ])rimipal  focus  of  the  eye  lies  back  of  the  retinal  plane, 
so  that  neutral  li^hl  waves,  instead  of  focussing  on  the  retina, 
fall  on  the  retina  in  dillusion  circles  of  inifocnssed  waves.  In 
other  words,  the  focal  length  of  tlie  refracting  system  of  the 
eye  is  greater  than  the  .mtt-ro-posterior  diameter  of  the  eye- 
ball. 


HYPEROPIA  189 

The  only  kind  of  light  wave  that,  falling  on  the  cornea,  could 
focus  upon  the  retina  of  a  hyperopic  eye  at  rest,  would  be  a 
convergent  wave ;  and  this  wave  would  have  its  source  of  origin 
beyond  infinity.  This  is  equivalent  to  saying  that  the  far 
point  of  a  hyperopic  eye  lies  beyond  infinity.  There  is,  how- 
ever, no  way  of  measuring  or  expressing  a  linear  distance 
beyond  infinity.  We  are  therefore  reduced  to  the  necessity  of 
defining  the  hyperopic  far  point  in  terms  of  the  reciprocal  of 
the  convergent  light  wave  that  is  normal  to  the  eye  at  rest. 

Further,  such  a  wave  could  not  have  its  source  of  origin  in 
nature,  since  all  light  waves  in  nature  are  divergent.  Nor  can 
such  a  wave  be  measured,  as  to  radius  and  curvature,  from  its 
imaginary  point  of  origin ;  first,  because  there  is  no  such  point 
in  space  as  "beyond  infinity,"  and  second,  because  a  converg- 
ent wave  does  not  travel  from  a  point  at  all,  but  to  a  point. 
Its  radius  and  curvature,  therefore,  must  be  determined  in 
relation  to  the  point  (back  of  the  eye)  to  which  it  is  converg- 
ing. 

Thus,  if  we  assume  an  eye  to  be  2  D.  hyperopic,  such  an 
eye  at  rest  could  focus  upon  its  retina  only  a  light  wave 
which,  when  it  falls  upon  the  cornea,  has  a  minus  curvature  of 
2  D.,  i.  e.  is  2  D.  convergent.  Such  a  wave  would  have  a 
radius  of  50  cm.  So  we  say  that  the  far  point  of  this  eye  is 
50  cm.  beyond  infinity;  but  this,  of  course,  expresses  simply 
the  convergence  of  the  wave  proceeding  from  the  eye's  far 
point.  Or,  we  may  say  that  the  far  point  lies  50  cm.  back  of 
the  cornea ;  which  signifies  the  same  thing. 

Clinically  the  hyperopic  far  point  is  determined  by  the  meth- 
ods which  have  already  been  described  in  the  chapter  on 
Accommodation,  that  is  to  say,  subjectively  by  means  of  the 
Snellen  type-chart,  objectively  by  means  of  static  retinoscopy. 

If  the  patient  is  able  to  read  No.  6  type  at  6  meters,  or  No. 
20  at  20  feet,  we  conclude  that  he  is  either  emmetropic  or  else 
hyperopic.  A  myope  could  not  read  this  type  at  this  distance, 
his  far  point  being  inside  of  infinity.  If  he  is  an  emmetrope, 
he  is  reading  20/20  with  his  eye  at  rest,  i.  e.  with  no  accommo- 
dation in  force.  If  a  hyperope,  he  is  using  some  accommoda- 
tion to  read  it.  To  determine  which  of  these  two  possibilities 
is  the  actual  state  of  affairs,  we  place  before  the  patient's  eye 


190  HYPEROPIA 

a  weak-power  comex  lens,  say  a  plus  .50.  This  makes  the 
light  waves  falling  on  the  patient's  cornea  slightly  convergent. 
If  he  be  an  emmetrope,  with  his  ciliary  already  relaxed  to  the 
limit,  he  has  no  way  of  adapting  his  eye  to  these  slightly  con- 
vergent weaves,  hence  the  20/20  type  blurs.  But  if  he  be  a 
hyperope,  reading  20/20  with  some  of  his  accommodation  in 
force,  he  can  readily  adapt  his  eye  to  the  slightly  convergent 
waves  by  simply  relaxing  .50  D.  of  his  accommodation.  The 
plus  .50  D.  lens,  therefore,  will  not  blur  the  type ;  and  the 
amount  of  plus  power  we  can  add  to  his  eye  without  blurring 
20/20  will  represent  the  amount  of  his  hyperopia  and  render 
him  normal. 

Objectively,  we  place  a  plus  lens  before  the  patient's  eye  and 
with  our  retinoscope  find  the  point  of  reversal.  In  hyperopia, 
with  the  ciliary  relaxed,  w-e  shall  always  find  this  point  of 
reversal  further  from  the  eye  than  the  focal  length  of  the  plus 
lens  employed.  The  difference  between  the  place  where  it 
ought  to  be  and  the  place  where  it  is,  in  terms  of  dioptrism, 
gives  us,  also  in  terms  of  rccijirocal,  the  patient's  far  point. 

Thus,  with  a  plus  2  lens  we  find  the  point  of  reversal  at  1.50 
meter.    It  ought  to  be  at  50  cm.    The  formula  will  be : 
1  1  1 


.50  1.50  75 

That  is  to  say,  the  patient's  far  point  is  75  cm.  bcyonil  infinity, 
and  he  is  1.33  D.  hyperopic. 

SOURCES   OF   ERROR. 

If  in  these  two  procedures  everything  worked  out  as  clearly 
in  practice  as  it  does  on  paper,  no  further  demonstration  would 
be  needed  of  the  patient's  hyperopia  or  of  his  distance  cor- 
rection. Unfortunately,  however,  such  is  not  the  case.  There 
arc  usually  complicating  factors  which  \itiatc  the  validity  of 
these  simple  tests,  chief  among  which  i.s  the  existence  of  a 
ciliary  sjiasm. 

By  dint  of  long-continued  use  of  his  accommodation  at 
infinity  the  hyperope  often  develops  a  permanent  contracture 
of  the  ciliary  muscle  wliicli  locks  his  accommodation  at  that 
j)oint,  so  that  he  cannot  relax  and  release  it.  Such  a  patient 
can  read  20/20,  but  vu  llic  addition  of  a  weak  plus  lens  he  is 


HYPEROPIA  I9i 

unable  to  give  up  any  of  the  accommodation  with  which  he 
was  reading  the  20/20,  and  the  lens  blurs  his  vision.  Under 
these  circumstances  he  is  quite  likely,  under  the  subjective 
test,  to  be  mistaken  for  an  emmetrope.  Occasionally  a  hyper- 
ope  who  works  habitually  within  a  range  less  than  infinity  will 
develop  a  spasm  w^hich  locks  his  accommodation  at  a  point 
nearer  than  infinity,  in  which  case  he  cannot  read  20/20,  and 
behaves  like  a  myope. 

The  same  condition  of  affairs  will,  of  course,  vitiate  the 
objective  test  with  the  retinoscope.  With  the  accommodation 
locked  at  infinity,  the  point  of  reversal  is  found  at  the  focal 
length  of  the  objective  lens,  simulating  emmetropia.  When 
locked  at  a  point  within  infinity,  the  point  of  reversal  is  found 
inside  the  focal  length  of  the  lens,  simulating  myopia.  This 
latter  condition,  known  as  "false  myopia,"  has  not  infrequently 
led  the  practitioner  to  prescribe  minus  lenses  for  a  hyperope. 

THE  FOGGING  SYSTEM. 

Various  plans  have  been  devised  for  dealing  with  this  situa- 
tion and  avoiding  the  source  of  error  which  it  involves.  The 
effectiveness  of  these  devices  rests  upon  one  of  two  principles 
of  action — either  they  must  force  the  patient  to  surrender  his 
spasm,  or  they  must  measure  and  reckon  with  the  amount  of 
the  spasm. 

Subjectively,  the  most  commonly  used  method  is  what  is 
known  as  the  "fogging  system,"  which  consists  in  placing 
before  the  eye  a  strong  plus  lens,  which  renders  the  patient 
highly  myopic,  and  leaving  it  there  for  a  while.  In  his  effort 
to  see  through  this  lens,  the  patient  is  induced  to  make  ex- 
traordinary effort  to  relax  his  ciliary ;  and  in  cases  where  the 
spasm  has  not  become  absolutely  fixed  this  plan  often  succeeds 
in  inducing  a  relaxation.  In  testing  with  the  Snellen  chart, 
the  strong  plus  lens  is  left  in  position,  and  gradually  reduced 
with  minus  power  until  the  patient  reads  20/20. 

Valuable  as  this  procedure  is,  it  still  leaves  us,  at  the  con- 
clusion, uncertain  as  to  whether  all  the  spasm  has  been  re- 
leased. Many  practitioners,  however,  content  themselves  with 
the  outcome  of  the  fogging  test  for  the  prescription  of  initial 
distance  correction,  and  trust  to  the  wearing  of  these  glasses 


192  HYPEROPIA 

to  presently  break  up  the  remainder  of  the  spasm,  so  that  the 
patient  may  at  some  future  time  be  given  his  full  correction. 

The  fogging  system,  as  applied  to  skiascopy,  is  substantially 
the  same  in  principle  and  technique.  Plus  spherical  power  is 
put  in  front  of  the  eye  under  test  until  it  is  fogged ;  then  minus 
spheres  are  used  until  the  shadow  is  abolished.  To  make  sure 
the  point  of  neutralization  has  not  been  passed,  the  observer 
draws  back  a  trifle  to  see  if  the  motion  is  then  against  the 
mirror.  With  this  method  the  distance  of  the  fixation'  object 
is  immaterial.  It  need  not  be  at  infinity.  The  patient  may  look 
at  a  line  of  letters  on  the  retinoscope,  the  same  as  in  dynamic 
skiacopy,  but  as  long  as  he  is  fogged  his  eyes  will  relax  fully. 

THE   HYPEROPE'S    ACCOMMODATION. 

The  mere  determination  of  the  far  point,  however,  no  matter 
how  accurately  it  be  done,  is  but  a  small  part  of  the  practical 
problem  of  the  refraction  and  correction  of  hyperopia.  The 
crux  of  the  hyperope's  condition  and  treatment  is  the  state  of 
his  accommodation  and  its  relation  tt)  his  convergence. 

The  first  thing  to  be  ascertained  is  the  patient's  available 
amplitude  of  accommodation,  which,  as  previously  set  forth, 
(see  Accommodation)  is  to  be  arrived  at  by  finding  his  near 
point  and  subtracting  the  far  point  from  it  in  terms  of  diop- 
trism.  There  are,  as  we  have  seen,  two  methods  of  determin- 
ing his  near  i)oint — subjectively  by  means  of  the  Jaeger  type 
charts,  and  objeclixcly  by  dynamic  skiascopy.  In  the  former 
test  we  find  the  nearest  point  at  which  the  patient  can  read  the 
type  of  appropriate  size  for  the  dist:ince  in  (|ucstion — i.  e.  as 
we  move  the  card  in  from  7?  cm.  to  50  cm.  we  must  shift  the 
patient's  attention  from  the  .75  type  to  the  .50  type,  and  so  on. 
In  the  latter  test,  with  the  patient's  vision  fixed  on  the  type  at 
the  top  of  the  retinoscope,  we  find  the  point  of  reversal. 

If  tlure  is  no  spasm  ])resent — or  none  luit  what  the  patient 
readily  surrenders — the  subjective  and  objective  fuulings  will 
approximately  coincide.  If  there  be  consiilerable  discrepancy 
between  them — the  ol)jective  near  point  being  considerably 
closer  in  than  the  subjective— it  is  generaly  considereil  that  the 
dilYcrence  represents  the  auKJunt  of  latent  hyperopia  present — 


I' 


HYPEROPIA  193 

although  many  excellent  authorities  dispute  this.  At  all 
events,  whatever  may  be  the  precise  connotation  of  the  dis- 
crepancy, it  seems  to  be  pretty  well  established  by  clinical 
experience  that  the  showing  of  a  marked  discrepancy  indicates 
the  existence  of  a  spasm. 

Where  no  discrepancy  shows — where  the  subjective  and 
objective  near  points  approximately  coincide — the  case  is  com- 
paratively simple.  The  hyperopia  is  wholly  manifest  and  the 
relation  between  accommodation  and  convergence  is  probably 
normal — i.  e.  normal  to  the  amout  of  hyperopia.  The  difter- 
ence  between  the  demonstrated  far  point  and  the  demonstrated 
near  point,  in  terms  of  reciprocals,  represents  the  available 
amplitude  of  accommodation ;  and  if  this  amplitude  be  what 
one  would  expect  at  the  age  of  the  patient,  the  case  is  one  of 
clear  sailing. 

Thus,  if  we  find  the  far  point  to  be  50  cm.  beyond  infinity, 
and  the  near  point,  both  by  subjective  and  by  objective  test,  to 
be  at  12.50  cm.,  in  a  patient  35  years  of  age,  then,  according 
to  the  formula : 

1  1  1 


12.50         .50         16.66 
the  patient's  amplitude  of  available  accommodation  is  6  D., 
which,  at  the  age  in  question,  is  normal.    This  patient  is  2  D. 
hyperopic,  and  a  correcting  lens  of  2   D.  ought  to  be  satis- 
factory. 

It  is  seldom,  however,  that  a  case  of  hyperopia  works  out 
with  such  nicety.  Continued  over-use  of  the  accommodation 
and  suppression  of  the  convergence  have  practically  always 
set  up  a  vicious  circle  of  troubles  in  and  between  these  two 
functions. 

ACCOMMODATION-CONVERGENCE  RELATION. 

It  has  already  been  stated  that  accommodation  normally 
stimulates  the  act  of  convergence ;  and,  as  the  hyperope  is 
obliged  to  accommodate  for  objects  at  infinity,  he  is  stimulated 
to  exercise  his  convergence  to  the  extent  of  his  accommoda- 
tion. This,  however,  he  may  not  do  under  penalty  of  double 
vision.  He  is  therefore  put  to  the  necessity  of  suppressing  his 
convergence. 


194  HYPEROPIA 

Whether  he  suppresses  it  in  potentia  or  in  esse.  i.  c.  whether 
he  sup])resses  the  desire  to  converge  and  withholds  the  inner- 
vation from  his  internal  rectus  muscles;  or  whether  the  inter- 
nal recti  are  inni-rwited  to  contraction  and  then  ccjunteracted 
by  an  equal  innervation  and  contraction  of  externals ;  is  a 
question  that  cannot  be  definitely  answered.  The  probability 
is  that  in  some  cases  the  former  course  is  followed  while  in 
others  the  latter. 

It  is  a  well-recognized  fact  that  patients  with  low  and  mod- 
erate degrees  of  hyperopia  suffer  more,  as  a  rule,  from  head- 
ache and  asthenopia  than  those  with  high  degrees  of  error. 
These  can  hardly  be  ciliary  symptoms ;  if  they  were,  the  re- 
verse would  be  the  case,  for  high  hyperopes  over-work  their 
ciliaries  more  than  low  hyperopes.  Most  likely  the  low-degree 
and  high-degree  hyperopes  choose  different  ways  of  dealing 
with  the  accommodation-con\ergence  disturbance,  the  former 
yielding  to  the  accommodation  stimulus  to  convergence  and 
counteracting  it  with  their  externrd  recti,  (hence  their  muscu- 
lar symptoms)  the  latter  suppressing  the  stimulus  altogether 
and  presently  learning  to  ignore  it. 

In  either  or  any  case  the  hyperoi)e  holds  his  convergence — 
or,  rather,  his  ])arallelism  at  infinity — as  a  condition  of  un- 
stable equilibrium,  in  virtue  of  his  desire  for  singleness  of 
vision;  and  every  hyperope  has  a  potential  esophoria  (whether 
he  manifests  it  or  not)  normal  to  his  hyperopia,  i.  e.  of  the 
same  meter  angles  as  the  dioptres  of  his  crrtjr,  or  api>roximately 
three  times  as  many  prism  dioptres. 

Such  a  state  of  affairs  is  morally  bound,  sooner  or  later,  to 
produce  a  serious  (lisrui)lion  of  the  coordination  between  ac- 
commodation and  con\ergence,  resulting  not  so  much  in  an 
insufficiency  as  in  an  ineflicieney  of  both  functions.  The  jtosi- 
tive  relative  accommodation  is  diminished;  relative  converg- 
ence is  exaggerated;  and  manifest  esophoria  is  almost  certain 
to  ensue.  Near  vision  is  therefore  the  bete  iioir  of  hyperopia, 
and,  as  previously  stated,  the  correction  of  distance  \ision  is 
usually  but  a  small  ])art  of  its  treatment.  The  accommodation- 
convergence  relation  calls  for  the  most  searching  investiga- 
tion. 


HYPEROPIA  195 

EXAMINATION. 
The  procedure  to  be  followed  in  the  examination  of  a  hyper- 
ope,  then,  may  be  outlined  as  follows : 

1.  Determine  the  far  point  subjectively,  by  means  of  the 
Snellen  chart,  preferably  by  the  fogging  method,  reducing  the 
high  plus  power  by  means  of  minus  spheres  until  20/20  is 
read.  The  net  plus  power  before  the  eye  at  this  point,  divided 
into  unity,,  represents  the  distance  of  the  far  point  beyond 
infinity. 

N.  B.  The  wheel  chart  should  be  introduced  into  this  test 
as  we  go  along,  for  the  purpose  of  detecting  or  excluding 
astigmatism.  The  necessity  of  watching  for  astigmatism  ap- 
plies to  every  form  of  procedure. 

2.  Fix  the  far  point  by  static  retinoscopy,  either  by  adding 
plus  neutralizing  power  until  the  ''with"  shadow  is  abolished, 
or  by  first  fogging  with  strong  plus  lens  and  then  reducing 
with  minus  power  until  the  "against"  movement  is  neutralized. 
The  net  result  of  this  static  retinoscopic  test  should  correspond 
with  the  subjective  finding. 

3.  Find  the  near  point  by  means  of  the  Jaeger  test  type. 

4.  Check  the  subjective  finding  of  the  near  point  by  dynamic 
skiascopy,  i.  e.  find  the  point  of  reversal  with  the  patient's 
accommodation  fixed  on  the  type  attached  to  the  retinoscope. 

The  near  point,  divided  into  unity,  gives  the  total  amount  of 
accommodation,  from  which  the  reciprocal  of  the  far  point 
must  be  subtracted  in  order  to  determine  the  available  accom- 
modation. 

If  the  findings  of  tests  Xos.  1,  2,  3,  and  4  approximately 
agree;  that  is  to  say,  if  the  subjective  and  objective  far  point 
and  near  point  are  substantially  the  same ;  and  if  the  ampli- 
tude of  available  accommodation  which  they  yield  is  normal 
to  the  patient's  age ;  then  the  probability  is  that  we  have  to 
do  with  a  case  of  simple,  uncomplicated  hyperopia,  which 
proper  distance  correction,  to  be  worn  constantly,  will  ade- 
quately remedy. 

If,  on  the  other  hand,  the  findings  do  not  coincide ;  if  the 
subjective  and  objective  far  point  and  (more  particularly)  near 
point  show  marked  discrepancy ;  and  if  the  available  accom- 


196  HYPEROPIA 

modation  is  below  iKMinal ;  then  it  is  almost  certain  that  the 
accommodation-convergence  relation  is  seriously  deranged,  and 
we  must  proceed  to  in\estigate  this  relation  by  the  following 
further  steps : 

5.  Test  out  the  relative  accommodation,  positive  and  nega- 
tive, by  the  gradual  addition  of  plus  and  minus  lenses,  respec- 
tively, while  the  patient  maintains  his  convergence  at  a  point, 
say,  33  cm.  distant.  Normally,  he  should  be  able  to  tolerate 
2  or  3  dioptres  each  way  and  still  be  able  to  read. 

6.  Determine  the  "comfortable  near  point"  by  means  of 
Lockwood's  cross-cylinder  test. 

7.  Make  an  adduction  and  abduction  test  at  infinity  by 
means  of  prisms,  base  out  and  base  in,  respectively. 

8.  Make  a  muscle  test  for  imbalance  at  infinity,  by  means 
of  the  Maddox  rod. 

9.  Make  a  test  for  muscle  imbalance  at  33  cm.  by  means  of 
the  double  line  and  index  pointer. 

I^etails  of  the  technique  of  all  the  above  tests,  with  ample 
illustrations,  will  be  found  in  the  chapter  devoted  to  this  pur- 
pose; and  a  discussion  of  their  significance  will  be  foimd  in  the 
sections  to  which  ihey  belong,  i.  e.,  the  accommodation  tests 
in  the  chai)ter  on  Accommodation,  muscle  tests  in  the  chaj)- 
ters  on  Convergence  and  Heterophoria. 

Briefly  summarized,  test  No.  5  should  show  3  to  5  dioptres 
of  positive  relative  accommodation  and  2  or  3  dioptres  oi  nega- 
tive. Test  No.  7  should  show  a  ratio  between  the  adduction 
and  the  abduction  powers  of  apprdxiniatdy  three  to  one.  Tests 
Nos.  8  and  9  ought  not  to  reveal  any  greater  degree  of 
esophoria  than  is  normal  to  the  annnint  of  hyperopia,  i.  e.. 
approximately  three  times  as  many  prism  dioptres  of  esophoria 
as  there  are  dioptres  of  hyperopia.  Indeed,  it  t>ught  not  to 
disclose  as  much  as  that,  for  a  j)atienl  rarely  manifests  all  of 
his  esophoria.  .And  tin-  amount  of  esophoria  should  register 
about  the  sanu'  at  iiilniity  and  near  point. 

TKEATMENT. 
If,  as  will   piitbably   liaiipen,   the  results  of  the  abo\e   tests 
disclose  marked   abnormalities,  the  situation   will   call   for   the 
nicest  judgnu-nl  of  tlu-  i)raitilitiner.     .So  far  as  distance  correc- 


HYPEROPIA  197 

tioii  is  concerned,  the  matter  is  comparatively  simple.  The 
hyperopic  patient  should  be  given  as  much  plus  correction  as 
he  will  tolerate,  for  constant  wear.  The  question  of  near  vision 
constitutes  the  crux  of  the  problem.  Xo  set  rules  of  thumb 
can  be  laid  down  for  its  solution.  Each  case  is  a  law  to  itself. 
Only  a  fev/  very  general  principles  can  be  enunciated  for  guid- 
ance in  shaping  one's  course. 

The  blanket  principle  which  governs  in  the  management 
of  all  cases  is  that  the  need  of  interference  depends  upon  the 
degree  of  flexibility  shown  by  the  patient's  functional  mechan- 
ism, and  his  ability  to  control  it. 

One  of  the  elemental  conditions  of  flexibility  is  an  adequate 
reserve.  Here,  then,  is  a  starting  place  from  which  to  reach 
a  decision  in  each  case.  The  most  extreme  and  obvious  ex- 
emplification of  this  working  principle  is  seen  in  a  case  where 
the  patient's  amplitude  of  accommodation  is  just  sufficient  to 
compass  the  point  at  which  he  wishes  to  do  his  near  work.  In 
such  a  case  there  is  no  reserve  at  all  at  that  point ;  positive  rela- 
tive accommodation  is  zero ;  there  can  be  no  flexibility ;  ac- 
commodation and  convergence  cannot  coincide  at  the  point  in 
question  ;  nor  can  the  patient  hold  such  a  near  point.  It  is  clear 
that  in  a  case  of  this  kind  we  shall  have  to  supply  enough  lens 
assistance  to  release  the  patient's  accommodation  to  a  suffi- 
cient amount  to  restore  something  like  a  normal  ratio  between 
amplitude,  near  point  and  reserve.  And  the  working  principle 
is  the  same  in  less  extreme  cases. 

The  adjustment  of  the  reserve,  however,  by  no  means  dis- 
poses of  the  problem.  There  are  more  cases  of  accommodative 
inefficiency  than  of  insufficiency.  Many  patients  whose  am- 
plitude and  reserve  are  normal — or  have  been  rendered  so  by 
lens  aid — are  still  unable  to  "make  ends  meet,''  so  to  speak,  and 
to  achieve  a  comfortable  co-ordination  between  accommodation 
and  convergence  at  the  point  at  which  they  must  work.  This 
is  probably  the  most  important  part  of  the  problem  in  every 
case  of  hyperopia,  and  on  its  successful  outworking  depends 
the  satisfrxtory  fitting  of  the  eyes.  We  have  already  dis- 
cussed this  matter  fully  in  the  chapters  on  Accommodation 


198  HYPERPHORIA 

and  Dynamic  Skiascopy,  respectively,  to  a  careful  perusal  of 
which  we  refer  the  student. 

Hyperphoria.  Muscular  iml)alance  in  which  the  eye  tends  to 
turn  upwaid.     (See  Heterophoria.) 

Hyperplasia.     Excessive  tissue   formation. 

Hypertension.  Excessive  tension.  Specifically  used  to  indicate 
hii^h  blood  pressure. 

Hypertrophy.    Abnormal  increase  in  the  size  of  an  organ. 

Hypertropia.     Squint  in  which  one  eye  turns  upward. 

Hyphemia.     Extravasation  of  blood  into  the  eye. 

Hypometropia.    'Jhe  opposite  of  hyj^ermetropia,  i.  e.,  myopia. 

Hypophasis.    Eyes  half  shut,  only  the  whites  l)eing  disclosed. 

Hypophoria.     Muscular  imbalance   downward.    See   Cataphoria. 

Hypopyon.    Pus  in  the  anterior  chamber. 

Hyposcleral.   Under  the  sclerotic  coat  of  the  eyeball. 

Hypotonia.  Decreased  intra-ocular  tension.  Also  written  Hypo- 
tonus  and  Hypotony. 

Hypotropia.    Dcxialion  of  the  axis  of  one  eye  downward. 

Identical  Points.  Corresponding  pcjints  in  the  two  retinae  on 
which  ra}  s  from  an  ol)ject  fall,  producing  single  binocular 
vision.     See  Binocular  Vision. 

Illaqueation.  'rrcatmcnl  of  ingrowing  lashes  by  drawing  them 
with  a  looj). 

Illumination.  All  the  pl.-ins  and  di-x  ices  that  e\  er  ha\e  been  or 
ever  can  be  propounded  for  the  best  lighting  of  rooms  resolve 
themselves  into  the  basic  principle  that  light  is  for  the  piu"- 
pose,  not  of  calling  .itli-iition  to  itself,  but  of  re\ealing  objects. 
The   wlujle   piolileni   of   lighting   consists  in   attaining   a   light 


1 


ILLUMINATION  199 

that  shall  obtrude  itself  least  upon  the  eye  and  at  the  same 
time  disclose  the  objects  to  be  viewed  with  the  greatest  pos- 
sible clearness. 

The  application  of  this  principle  necessarily  implies  the  re- 
flection of  the  light  from  the  object  to  the  observer's  eye.  Its 
working  out,  therefore,  involves  the  consideration  of  two  feat- 
ures:  (1)  the  source  and  nature  of  the  light,  and  (2)  the  course 
to  be  traversed  by  the  light  from  its  origin  to  the  object  and 
thence  to  the  observer.  All  questions  concerning  the  light 
come  under  the  head  either  of  its  illuminating  qualities  or  of  its 
propagation. 

Dififusion.  In  fulfillment  of  the  requirement  that  light  shall 
attract  as  little  attention  as  possible  to  itself,  it  is  essential 
that  it  be  as  diffuse  as  possible.  Daylight  (but  not  direct  sun- 
light) is  the  ideal  illumination,  in  spite  of  all  the  foolish  asser- 
tions to  the  contrary,  and  it  should  be  admitted  to  the  room 
in  such  a  way  as  to  render  its  distribution  as  diffuse  and  as 
uniform  as  possible. 

Daylight  being  inaccessible,  or  for  any  reason  undesirable, 
the  next  best  substitute  is  the  indirect  system  of  lighting  in 
which  the  light  is  reflected  from  the  ceiling  and  walls  of  the 
room.  After  that,  the  next  best  is  a  white  or  slightly  yellow 
artificial  light  surrounded  by  a  spherical  globe  of  frosted  glass 
for  the  purpose  of  dift'usion.  The  globe  need  not  be  a  com- 
plete sphere,  for  it  is  rarely  necessary  to  direct  light  upward, 
but  it  should  be  of  a  spherical  curvature,  so  as  to  give  uniform 
diffusion.  Uniformity  of  distribution  is  best  attained  by  a 
uniform  distribution  of  the  sources  of  light  throughout  the 
room,  rather  than  grouping  them  in  the  center.  This  must  of 
course  be  done  with  due  regard  to  the  matter  of  intensity  and 
of  the  purposes  for  which  the  room  is  to  be  used. 

Intensity.  The  intensity,  or  what  the  layman  calls  the 
strength  of  the  light,  is  a  matter  whose  adjustment  depends 
upon  the  purpose  for  which  it  is  wanted,  and  one  in  which  the 
most  absurd  paradoxes  are  perpetrated  in  actual  practice.  One 
does  not  require  as  much  intensity  of  light  for  a  dancing  party 
or  a  reception  as  for  reading  or  sewing;  yet  as  a  rule  the  danc- 


200  ILLUMINATION 

ing  hall  or  reception  room  is  lighted  far  more  brilliantly  than 
the  reading  or  sewing  room.  Too  great  intensity  of  light  is  in- 
jurious to  the  eyes  by  reason  of  its  exhausting  effects  upon  the 
retina,  and  also  because  it  rexeais  more  detail  than  is  neces- 
sary and  thus  induces  nagging  eye-strain.  Too  little  intensity, 
on  the  other  hand,  is  harmful  because  it  necessitates  closer 
application. 

It  is  estimated  that  for  ordinary  visual  purposes,  e.  g.,  for  a 
dining  or  reception  room,  an  intensity  of  32  candle  power  per 
1000  cubic  feet  of  space  is  about  right,  while  for  close  work  it 
should  reach  64  candle  power  per  1000  cubic  feet. 

Modes  of  Lighting.  As  stated,  daylight  is  easily  the  most 
desirable  mf)de  of  light.  In  addition  to  its  other  superiorities, 
it  has  the  ad\antage  of  furnishing  wliite  light,  which  is  most 
c(jnducive  to  visual  acuity.  Its  chief  disadvantages  are  its 
unre]ial)ility,  short  duration,  and  the  difficulty  of  admitting  it 
in  sufficient  quantities. 

Artificial  modes  of  light  are  desirable  in  proportion  as  they 
approximate  the  advantages  of  daylight  and  compensate  for 
its  disadvantages. 

Electric  Light.  The  mode  of  artificial  light  which  comes 
nearest  to  fulfilling  these  conditions  is  undoubtedly  the  electric 
incandescent  light.  It  gives  an  almost  white  light;  it  burns 
with  comparative  steadiness;  it  yields  the  greatest  degree  of 
intensity  for  the  least  bulk  ;  its  intensity  is  most  easily  meas- 
ured and  reguhited  ;  it  does  not  x'itiate  the  air;  does  not  affect 
humidity  or  temperature  to  any  marked  extent ;  and  it  carries 
no  danger  of  fire.  Incidentally  it  ma)  be-  .'idded  that  it  is  the 
most  economical  light,  both  in  respect  of  ])roduction  and  of 
l)hysical  lu-alth. 

(las.  Next  to  the  electric  light,  illuminating  gas,  with  some 
form  (jf  mantle  burner,  is  most  desirable.  \\\  means  of  these 
burners  much  of  the  objection  to  the  color  and  unsteadiness 
of  the  llame  is  overcome.  There  still  reni.iin,  ho\\i'\i-r,  the  ob- 
jections of  vitiating  the  atmosphere,  raising  tempi'rature,  low- 
ering hmnidity.  and  risk  of  lire,  to  which  m.iy  be  added  the 
dilticult)'  ol  (ihlaiiiinj;  ;i  i^ood  ^rade  of  ^as.  .Acetyleni'  is  better 
than  coal  gas. 


ILLUMINATION  201 

Oil  Lamps.  In  the  absence  of  facilities  for  electric  light  or 
gas,  one  must,  of  course,  use  oil  lamps.  They  are  very  un- 
desirable things  at  best,  and  all  one  can  do  is  to  reduce  their 
objectionable  features  as  much  as  possible,  by  employing  a 
high  grade  of  kerosene,  i.  e.,  one  with  a  high  flash  point,  a 
well-made  air-tight  lamp,  a  good  circular  wick,  and  a  first-class 
chimney  surrounded  by  a  white  opaque  globe.  An  ordinary 
sized  room  will  require  two  or  three  such  lamps,  of  large  size, 
to  light  it  adequately. 

Position  of  Light.  The  object  to  be  viewed, — the  book  to 
be  read,  for  example, — should  not  be  between  the  person  and 
the  source  of  light ;  in  fact,  the  source  of  light  should  not  be 
in  front  of  the  observer  at  all.  For  a  light  in  front  calls  atten- 
tion to  itself,  exhausts  the  retina,  is  not  reflected  from  the 
object  and  therefore  does  not  illumine  it,  and  in  the  case  of 
writing  or  sewing  casts  a  shadow  toward  the  worker — in  all 
of  which  it  violates  the  canons  of  good  illumination. 

It  is  equally  evident  that  the  source  of  light  should  not  be 
immediately  behind  the  observer,  for  then  his  own  body  will 
intercept  the  light  so  that  it  cannot  reach  the  object,  and  all 
the  room  will  be  illumined  except  the  very  portion  where  illum- 
ination is  desired. 

The  source  of  light  directly  to  the  side  of  the  observer  is 
almost  as  bad  as  directly  in  front ;  for  in  this  position  the  rays 
strike  the  object  at  an  extremely  oblique  angle,  and  are  re- 
flected at  an  equally  oblique  angle  to  the  other  side ;  many  of 
them  pass  laterally  between  him  and  the  object.  In  addition 
to  these  faults  of  reflection,  the  light  strikes  his  eye  at  a  lateral 
inclination,  which  is  peculiarly  irritating,  as  everyone  knows 
who  walks  sideways  to  the  sun  when  it  is  just  going  down. 

The  Correct  Position.  The  most  satisfactory  position  for 
the  light  is  midway  between  the  rear  and  the  left  lateral.  In 
this  position  the  light  falls  on  the  object  at  a  slightly  oblique 
angle  which  just  reflects  it  into  the  observer's  eye,  and  no 
shadow  is  cast  between  the  observer  and  the  object.  The  right 
lateral  is  not  quite  so  good  a  position,  because  in  most  persons 
the  right  eye  is  the  dominant  eye. 

Influence  of  Position  on  Intensity.  In  arranging  such  a  posi- 
tion, the  light  must  not  be  too  far  from  the  object  to  illumine 


202  ILLUMINATION,  OPHTHALMIC 

it  in  sufficient  detail,  or  so  near  as  to  unnecessarily  exaggerate 
detail  and  tire  the  retina. 

Influence  of  Position  on  Diffusion.  The  same  remarks  apply 
to  the  diffusion  of  light.  If  the  source  of  light  be  too  near  the 
object,  the  light  will  not  be  sufficiently  diffused  by  the  time 
it  reaches  it;  if,  on  the  other  hand,  it  be  too  far  away,  dif- 
fusion will  be  so  great  as  to  weaken  intensity  and  reflecting 
power. 

What  hr.s  here  been  said  concerning  the  position  and  dis- 
tance of  the  source  of  light  applies  equally  to  windows  trans- 
mitting dayligiit  and  to  artificial  sources  of  light. 

Illumination,  Ophthalmic.  The  different  methods  of  illuminating 
in  ophthalmology  are  as  follow^s : 

Axial  illumination.  When  the  light  is  transmitted  along  the 
axis  of  a  lens,  or  reflected  along  the  axis  of  a  mirror. 

Direct  illumination.  When  the  light  is  thrown  directly  on 
to  the  object. 

Focal  illumination.  When  the  light  is  focussed  on  the  object 
by  lens  or  mirror. 

Obli(iue  illumination.  When  the  light  is  passed  oblicjuely 
through  a  lens  and  made  to  fall  on  one  side  of  the  object. 

Image.  An  optical  image  is  formed  by  tlie  reuniting  of  waves 
of  light  from  an  object,  by  means  of  a  lens  or  a  mirror,  in  a 
series  of  foci  identical  in  arrangement  and  (juality  with  those 
from  which  the  light  proceeded.  The  area  occupied  by  this 
group  of  foci,  i.  e.,  the  size  of  the  image,  has  nothing  to  do  with 
its  accuracy. 

The  only  kind  of  mirror  or  lens  tiiat  can  i)roduce  an  accur.ite 
image  is  a  spherical  ouf  ;  all  others  distort  the  contcnn-  of  the 
image.  I'urthermore,  every  real  image  is  inverted,  because 
the  nodal  point  of  a  spherical  foord  systini.  where  the  axial 
rays  cross,  always  lies  in  front  of  the  local  poiiu. 

Indistinct,  wh.at  ni.iy  \)v  called  near-im.ages.  ;ire  formed  by 
waves  that  are  .almost  focussed.  .Strictly  speaking,  from  an 
optical  standpoint,  these  are  not  images  at  all;  but  physiolo^- 
icalK  .  if  tlu-  waves  .in-  focussed  nc.irly  rnon}.;li  to  produce  tlu- 
reijuired   reaiti(»n   in   xisioii,  w  r  call   the  iiactioii  ;in   ini.'ij^e. 


IMAGE  203 

The  size  of  an  image  depends  upon  its  distance  from  the 
focussing  lens  or  mirror.  Its  size,  as  compared  to  that  of 
the  object,  is  in  direct  ratio  to  its  distance  from  the  lens  or 
mirror,  as  compared  to  that  of  the  object. 

For  further  details  of  images  and  their  optical  relationships, 
see  Lens. 

Aerial  Image.  One  that  is  formed  in  the  air,  such  as  we 
see  in  indirect  ophthalmoscopy.  Such  an  image  is  not  really 
seen,  as  images  are  only  visible  through  the  intervention  of  a 
screen.  An  aerial  image  becomes  an  object,  which  is  again 
imaged  on  the  retina,  and  the  retinal  image  perceived. 

After-image.     See  After-image. 

Chiasmal  Image.     See  Chiasmal  Image. 

Cyclopic  Image.  The  single  image  of  one's  eye  which  one 
sees  when  looking  stereoscopically  into  a  plane  mirror,  i.  e., 
with  each  eye  fixed  upon  the  corresponding  eye  in  the  mirror. 

Direct  Image.    Same  as  an  upright,  or  virtual  image. 

Erect  Image.     Same  as  a  virtual  image. 

False  Image.  The  image  made  upon  the  retina  of  a  deviat- 
ing eye  in  strabismus  or  diplopia. 

Indirect  Image.    Same  as  an  inverted  image. 

Real  Image.  An  image  produced  by  the  actual  focussing 
of  light  waves  from  an  object  by  means  of  a  convex  lens  or  a 
concave  mirror.  Such  an  image  is  inverted,  because  the  focal 
point  is  always  posterior  to  the  nodal  point. 

Retinal  Image.  The  representation  of  an  object  formed 
upon  the  retina  by  the  focussing  of  rays  from  the  object  at  the 
retinal  plane.     Being  a  real  image,  it  is  inverted. 

Homonymous  Image.  An  image  which  is  projected  to  the 
same  side  of  the  visual  field  as  the  eye  which  perceives  it. 

Stereoscopic  Image.  A  single  image  formed  by  the  fusion 
of  the  two  retinal  images,  differing  slightly  from  each  other, 
so  as  to  give  the  sensation  of  depth.    See  Binocular  Vision. 

Upright  Image.     Same  as  a  virtual  image. 

\^irtual  Image.  An  imaginary  image  formed  by  the  projec- 
tion of  divergent  light  waves  from  a  concave  lens  or  a  convex 
mirror  to  their  apparent  point  of  origin.  Such  an  image,  as 
viewed  by  the  eye,  is  an  upright  one,  because  the  waves  have 
undergone  no  crossing. 


204  IMAGE  LINE 

Image  Line.  A  straig:ht  line  in  the  image-space  (q.  v.)  in  an 
infinitely  extended  plane,  in  any  meridian  containing  the 
optical  axis  of  a  lens-system. 

Image  Plane.  The  plane,  real  or  virtual,  perpendicular  to  the  axis, 
in  which  are  situated  the  constituent  points  of  an  image  pro- 
duced by  a  lens  or  a  mirror  system. 

Image  Point.  Any  one  of  the  constituent  points  in  the  image 
plane  as  described  above. 

Image  Space.  The  space  traversed,  actually  or  virtually,  by  the 
effective  rays  directed  toward  the  constituent  points  of  the 
image.     See  Collinear  Space  Systems. 

Images  of  Purkinje.  The  three  catoptric  images  produced  by 
the  surface  of  the  cornea  and  the  anterior  and  posterior  sur- 
faces of  the  crystalline  lens,  used  for  the  purpose  of  demon- 
strating the  presence  of  the  crystalline  lens,  and  for  showing 
tlie  l)ulging  of  its  anterior  surface  in  accommodation.  See 
Accommodation. 

Imbalance.     See  Heterophoria. 

Incident  Ray.  Term  applied  to  a  ray  of  light  before  it  strikes 
a  reflecting  medium  or  enters  a  refracting  one.  It  is,  of  course, 
a  geometrical  quantity  with  which  one  calculates  the  geomet- 
rical relations  of  rellection  and  refraction. 

Inclinometer.  A  patented  instrument  fur  measuring  the  inclina- 
tion of  the  visual  axes. 

Indirect  Vision.  The  \ision  wliicli  results  from  the  falling  of 
f(jcussed  rays  upon  the  peripheral  portions  of  the  retina.  .\1- 
tliough  much  less  acute  than  direct  \ision,  it  is  of  great  j^rac- 
tical  use  in  daily  life.     bOr  further  ])articulars  see  Perimetry. 

Index  of  Refraction.  The  formula  expressing  the  relati\  i-  density 
to  light,  and  therefore  the  ri'lati\e  id i acting  power,  of  a  sub- 
stance as  ctJtnpared  with  ;iir. 

l)istincti<tn  must  be  made  brtwcen  tlie  actual  <|uality   which 
tlu-  index  r(]»resents.  and  the  indi-x  itself.      The  actu.il  dilVerence 


INFINITY  205 

of  refractive  density  between  the  air  and  the  substance  in 
question  is  a  function  of  the  opposition  which  they  ofifer  to 
the  speed  of  the  light  waves.  But  the  index  by  which  this  dif- 
ference is  expressed  is  a  geometric  formula,  based  upon  the 
angles  which  the  incident  and  the  refracted  ray  make,  respec- 
tively, with  the  perpendicular  of  the  refracting  surface.  The 
index  is  as  follows : 

sine  of  angle  of  refracted  ray 


sine  of  angle  of  incident  ray 
Since  the  refracted  ra}-  makes  a  less  angle  with  the  perpen- 
dicular than  the  incident  ray  when  passing  from  a  rare  to  a 
denser  medium,  the  index  in  that  case  is  a  positive  quantity. 
When  passing  from  denser  to  rarer,  the  index  is  a  negative, 
or  minus  quantity. 

Infinity.  Actually,  of  course,  there  is  no  such  point  in  practical 
optics.  Light  waves  coming  from  such  a  point  would  have  no 
curvature,  but  be  neutral  waves.  However,  when  waves  have 
traveled  6  meters,  or  20  feet,  the  degree  of  curvature  they  pos- 
sess is  so  slight,  and  the  segment  of  the  wave  that  falls  upon 
the  eye  is  so  small,  that  it  is  regarded  as  a  neutral  wave,  and 
6  meters  or  20  feet  is  regarded  as  infinity. 

Infraduction.     Turning  of  the  eye  downward. 

Infraorbital.     Beneath  the  orbit. 

Infrared.  The  invisible  heat  rays  beyond  the  red  end  of  the 
spectrum. 

Instruments — Oculo  Refractive.  These  instruments  are  primarily 
designed  to  facilitate  the  work  of  the  operator  in  ocular  and 
refractive  diagnosis.  Hundreds  are  manufactured  and  the 
author  believes  that  every  one  of  them  has  some  intrinsic 
value.  Every  manufacturer  of  instruments  is  prepared  to  fur- 
nish descriptive  literature  pertaining  to  his  products,  going 
into  details  so  as  to  enable  the  operator  to  obtain  the  best 
results,  and  it  would  be  advisable  to  communicate  with  the 
manufacturers  whose  names  appear  in  connection  with  each 
instrument  mentioned. 


206 


INSTRUMENTS 


AXOMETER  (Lloyd's). 
This  instrument  is  tlesii^ned  to  (ktermine  instantly  the  axis 
of  a  lens.    The  focus  is  determined  at  the  same  time  and  with 


L  (jvil's    AxuiiKtcr. 

no  extra  manipuhition.  It  is  also  good  to  examine  complete 
work  and  broken  lenses.  Made  by  the  Globe  Optical  Com- 
pany, Boston,  Mass. 


HiiHx'tilui'  Curiifiil  AUiToBcopc. 


INSTRUMENTS  207 

BINOCULAR    CORNEAL    MICROSCOPE. 

This  corneal  microscope  gives  the  proper  stereoscopic  effect 
necessary  for  the  examination  of  the  cornea.  Owing  to  its 
penetrating  power,  deeper  layers  of  the  corneal  structure  are 
defined  simultaneously  with  the  surface,  enabling  the  operator 
to  follow  defective  or  injured  parts  of  the  cornea  to  a  consid- 
erable depth.  Supplied  mounted  on  a  round  base  or  tripod,  or 
with  the  base  of  the  instrument  taking  the  form  of  a  compound 
slide  arrangement.  Furnished  with  a  Gullstrand  Illuminating 
Attachment.  Made  by  the  Bausch  &  Lomb  Optical  Co.,  Roch- 
ester, N.  Y. 

BINOCULAR  MAGNIFIER. 

Used  by  surgeons,  eye,  ear,  nose  and  throat  specialists, 
dentists,  watchmakers,  botanists,  geologists  and  others  who 
must  examine  small  objects  with  comparatively  low  magnifi- 
cation. Due  to  an  arrangement  of  two  rhombohedric  prisms, 
it  gives  not  only  binocular  vision,  but  a  practically  natural 
perspective.  The  Binocular  Magnifier  is  furnished  in  magnifi- 
cations ranging  from  0.75  X  to  3  X  with  either  fibre  or  elastic 
headband,  both  adjustable.  Made  by  the  Bausch  &  Lomb 
Optical  Co.,  Rochester,  N.  Y. 


Binocular   Magnifier. 

CORNEAL  MICROSCOPE. 

Designed  by  Dr.  Harry  Cradle  for  diagnosing  various  cor- 
neal conditions,  such  as  ulcers,  etc.,  simplified ;  but  any  changes 
in  the  iris  or  reaction  of  the  pupil  are  immediately  manifest. 
The  instrument  is  an  attachment  for  any  of  the  Hardy  oph- 
thalmometers.   Made  by  F.  A.  Hardy  &  Co.,  Chicago. 


208 


INSTRUMENTS 


EXOPHTHALMOMETER. 
With  this  little  instrument  an  indixidual  examiner  is  able 
to  measure  exophthalmia  accurately  and  rapidly.  The  Ex- 
ophthalmometer  is  simple  in  construction  and  easy  to  use;  it 
is  so  made  that  the  outer  margins  of  the  orbital  cavities  of  the 
patient's  eyes  serve  as  the  base  and  support  of  the  instrument. 
Made  by  the  Bausch  &  Lomb  Optical  Company,  Rochester, 
N.  Y. 

GEM   AXIS   MARKER   AND   PRISM   MEASURE. 
An  instrument  for  centering  and  marking  lenses  of  all  kinds. 
Making  it  possible  to  measure  prisms     and     determine     the 


(iem  Axis  Marker  and   Prism  Mi-asiiro. 


amount  of  di-ccntcriiig  for  a  given  prism  strength.  Made  en- 
tirely of  metal  with  a  sliding  target  through  which  the  liKht  is 
secured,  making  it  possible  to  change  the  distance  between 
the  target  and  lenses  for  very  weak  or  very  strong  power.  Made 
by  the  ( ilube  (  )ptical  Co.,  I'oston.  Mass. 


INSTRUMENTS 


209 


INTERPUPILLARY  GAUGE. 
This  is  an  instrument  for  measuring  the  distance  between 
the  pupils  when  vision  is  directed  upon  distant  or  near  objects. 
It  is  very  simple  to  operate,  as  it  does  not  involve  any  setting 
of  scale.  It  measures  the  distance  of  either  pupil  from  the 
center  of  the  bridge  of  the  nose,  thereby  taking  into  account 
the  almost  universally  existing  asymmetry  of  the  eyes  with 
respect  to  that  point.  Made  by  the  Bausch  &  Lomb  Optical 
Co.,  Rochester,  N.  Y. 


Interpupillary  Gauge. 

KERATOMETER. 

The  Keratometer  enables  the  operator  to  measure  the 
diameter  of  the  cornea  quickly  and  accurately.  It  is  also 
adapted  for  measuring  the  distance  between  the  apex  of  the 
cornea  and  the  vertex  of  the  spectacle  lens.  Made  by  the 
Bausch  &  Lomb  Optical  Co.,  Rochster,  N.  Y. 

KERATOMETER   (SutclifFe). 

A  3  Mire  Ophthalmometer  which  automatically  gives  a  read- 
ing on  scales  after  the  focus  and  axis  are  determined. 

LENS  MARKING  INSTRUMENT  (Stoco). 
Designed  to  check  the  finished  work — detecting  and  indicat- 
ing the  amount  of  any  error  in  the  finished  lenses.  It  also  me- 
chanically lays  out  the  work  for  cutters  and  edgers.  A  handy 
instrument  for  refractionists.  Made  by  the  Standard  Optical 
Co.,  Geneva,  N.  Y. 


210 


INSTRUMENTS 


i 


Lens    Marking.    Centering    and    Testing  Instrument    (Stoco). 


OPHTHALMOMETER,  C.  I. 

For  detecting  the  presence  of  corneal  astigmatism,  measur- 
ing it  and  locating  the  principal  meridians.  The  principal  feat- 
ures of  the  latest  one-position  model  are  the  fixed  mires,  rotat- 


C.   I.  Ophtli;ilmomcii-i'. 


ing  cyci)icce  and  (loiililc  i^lass  prisms.     'I'lu'  advantage  claimi'd 
by  the  manufacturers  lii-s  principally  in  the  movable  eycpiccf 
which  elinunalcs  the  necessity  of  revolving  the  mires  to  mens 
ure  the  curvature  in  the  second  i)rinci[)al  uuridian.     Made  by 
V.  A.  llanlv  \'  (.(•.,  Chicago. 


INSTRUMENTS 


211 


OPHTHALMOMETER   (Meyrowitz). 
Designed  along  the    Javal-Schiotz    idea    and    including    a 
Wollaston  bi-refringent  quartz   prism   and  achromatic  objec- 


Moyrowitz    Ophthalmometer. 

lives.  The  latest  of  the  firm's  ophthalmometers  is  known  as 
the  Souter,  which  it  is  claimed  can  be  used  for  measuring  not 
only  the  astigmatism  of  the  cornea  but  also  the  anterior  and 
posterior  surfaces  of  the  lens.  Made  l:)y  E.  B.  Meyrowitz, 
New  York. 

OPHTHALMOMETER    (Universal). 
For  the  purpose  of  measuring  the  curvature  of  the  cornea  to 
ascertain  whether  corneal   astigmatism  exists  and  if  present, 


Universal  Ophthalmometer. 


212  INSTRUMENTS 

to  measure  tlie  deg^ree  of  error  and  to  locate  the  principal 
meridians.  The  distinguishing  features  of  this  type  of  ophthal- 
mometer are  movable  mires,  an  eyepiece  adjustable  f(jr  the 
operator's  spherical  error,  and  (piartz  Iji-refringent  i)risms  for 
which  the  manufacturers  claim  decided  advantages.  Made  by 
the  General  (Jptical  Co.,  Mt.  \'crnon,  N.  Y. 

OPHTHALMOSCOPE. 

For  examining  the  retina,  lens  and  media.  ( )pcrates  by  re- 
flecting light  into  the  eye.  The  source  of  the  light  in  some 
types  is  located  back  of  the  patient,  but  in  more  modern  and 
popular  models  the  light  is  contained  within  the  instrument 
and  is  electrically  operated.  The  distinctive  types  are  the 
Loring,  Hare  and  Morton,  between  which  the  difference  lies 
in  the  shape  of  the  instrument  proper  and  the  arrangement  of 
the  lenses.  There  are  two  general  types  of  mirrors,  i.  e.,  those 
having  circular  or  oval  peep-holes  and  those  having  U-shaped 
peep-holes,  the  latter  l)eing  known  as  the  M^^arple  type.  Manu- 
facturers of  ophthalmoscopes  in  the  United  States  are  De  Zeng 
Standard  Co.,  Camden,  N.  J.;  General  Optical  Co.,  Mt.  \'ernon, 
N.  Y.,  and  several  others. 

OPHTHALMOSCOPE  and  RETINOSCOPE  (Geneva). 
The  ophthalmoscope  and  retinoscope  is  constructed  to  ascer- 
tain the  refractive  and  pathological  condition  of  the  eye.  With 
the  ophthalmoscope   the  refractionist   is   enabled   to  obtain   a 
highly  magnified  and  clear  view  of  the  retina  in  true  colors 


Oi'neva   Ophtlialinosfoin'   arid 
Hot  iiioscopi'. 

and  proportions.  It  gives  a  broad  field  view.  The  illumination 
extends  o\  er  the  entire  retina  and  it  is  not  necessary  to  release 
the  accommodation  of  the  oi)eratur  or  the  jjatient.  The  special 
construction  of  llir  retinoscope  is  the  construction  of  the  mir- 
ror, and  witli  the  concealed,  cotu-cntrati-d  light  it  gives  a  clear 


INSTRUMENTS  213 

shadow  movement.  The  Hght  is  close  to  the  mirror,  so  a 
strong  light  is  not  needed.  The  instrument  presents  a  con- 
venient working  distance ;  sufficiently  great  to  separate  opera- 
tor from  patient  and  yet  close  enough  to  permit  the  operator  to 
reach  the  patient's  face  when  necessary.  Xo  dark  room  is 
necessary  for  the  operation  of  this  instrument.  Made  by  the 
Geneva  Optical  Co.,  Chicago. 

OPHTHALMOSCOPE  GULLSTRAND. 

The  large  Gullstrand  Ophthalmoscope  permits  an  accurate 
examination  and  observation  of  the  fundus  of  the  eye,  furnishing 
magnifications  of  from  5  to  40  times  for  monocular  observation, 
and  20  times  for  binocular  observation.  The  stereoscopic  effect 
obtainable  with  the  binocular  arrangement  is  excellent,  while 
the  field  of  view  is  brightly  illuminated  by  an  ingeniously  con- 
structed illuminating  device  supplied  with  the  ophthalmoscope. 

The  device  illustrates  "a"'  as  the  light  source,  "b"  an  aplanatic 
aspheric  lens,  "c"  the  slit  of  the  illuminating  system,  "d''  a  dia- 
phragm for  confining  the  field,  "e"'  a  second  aplanatic  aspheric 
lens,  "f"  a  non-silvered  glass  wedge,  "g"  the  ophthalmoscope 
lens  and  "h"  a  diaphragm  situated  in  front  of  the  objective  and 
forms  the  entrance  pupil  of  the  observation  system. 

A  speciallv  designed  drawing  apparatus,  by  means  of  which 
the  fundus  of  the  eye  under  examination  can  be  drawn  in  detail, 
can  be  attached  to  the  instrument,  while  a  special  ocular  is  fur- 
nished for  demonstration  purposes. 

Owing  to  the  ingenious  construction  the  manipulation  of  the 
apparatus  is  the  simplest  possible,  and  the  dimensions  of  the  in- 
strument enable  the  operator  to  keep  at  a  convenient  distance 
from  the  patient,  so  that  no  inconvenience  whatever  is  felt  dur- 
ing the  examination.  The  chin  rest  with  head  support  enables 
the  patient  to  rest  comfortably  and  to  keep  reasonably  steady 
during  the  examination,  thus  greatly  facilitating  continuous 
observation. 

PHORO-OPTOMETER. 

An  instrument  for  subjectively  measuring  the  refraction  and 
muscular  condition  of  the  eyes.  The  complete  instrument  con- 
sists of  a  battery  of  trial  case  lenses,  a  graduated  pupillary 
adjustmeni,  a  brow  rest,  Risley  30  diopter  double  rotary 
prisms,  Maddox  multiple  rods,  a  Stevens  phorometer  attach- 


214 


INSTRUMENTS 


ment,  a  spirit  level  and  an  adjustable  near  point  test.  The 
double  rotary  prism  unit  is  composed  of  two  15  D.  prisms 
mounted  back  to  back.    They  rotate  in  opposite  directions  and 


No.  560  De  Zeng  Phoro-Optometer. 

any  prism  equivalent  from  0  to  30  D.  may  be  obtained.  By 
turning  the  entire  unit,  the  prism  base  may  be  located  in  any 
meridian  of  the  circle.  The  instrument  can  be  had  with  a  floor 
stand,  wall  bracket  or  chair  and  table  attachment.  Made  by 
De  Zeng  Standard  Co.,  Camden,  N.  j. 
PHORO-OPTOR. 
A  new  mstrument  designed  by  the  General  Optical  Com- 
pany, Alt.  V^ernon,  N.  Y.,  for  ascertaining  the  refraction  and 
the  muscular  conditions  of  the  eye.  Consists  of  a  closed  in 
battery  of  spheres  and  cylinders  and  rotary  prisms.  Maddox 
Rods,  etc.     Su])plied  with  various  forms  of  fixtures. 

SKI-OPTOMETER. 

An  instruiiKMit  for  estimating  errors  of  n-fiaotioii  and  muscu- 
lar baianic.  it  consists  of  a  coinpk-tc  battery  of  Irnses  for 
each  eye  st-t  in  adjustable  trial  franii-  with  individual  i)upillary 
gauge,  combined  with  a  double  Ki^Uy  rotary  prism  for  each 
eye — a  red  Maddox  rcjd  for  the  left  eyi-  and  a  white  Maddox 
rod  for  the  rigbt  eye.     There  is  also  a  Ste\ens  phorometer  at- 


INSTRUMENTS  215 

tachment.      The    instrument    comes    with    floor     stand,     wall 


No.   215  Woolf  Ski -Optometer. 

bracket,  chair  attachment  and  table  clamp.     Manufactured  by 
the  A\"oolf  Instrument  Corp.,  New  York  City. 

STEVENS   PHOROMETER. 

This  instrument  was  invented  by  Dr.  Stevens  of  New  York, 
N.  Y.  It  is  composed  of  Maddox  rod  over  left  eye  and  prisms 
so  arranged  that  accurate  measurement  of  the  vertical  and 
lateral  muscles  can  be  made  by  having  the  patient  look  at  a 
spot  light  20  feet  distant.  The  principle  is  binocular.  Most  of 
the  optometers  are  equipped  with  this  instrument.  It  is  manu- 
factured by  the  De  Zeng  Standard  Co.,  Camden,  N.  J.,  and  the 
Woolf  Instrument  Corporation,  New  York  City. 

PERIMETERS. 

For  measuring  the  field  of  vision.  \'aluable  in  locating 
many  pathological  conditions  not  detected  readily  by  other 
means.  The  new  combined  electric  and  daylight  perimeter  is 
of  the  improved  McHardy  self-registering  type  and  contains 
Dr.  Black's  translucent  spectral  colors  in  addition  to  the  regu- 
lar daylight  test.  Known  as  No.  475  McHardy  Perimeter. 
No.  460  of  the  same  manufacture  is  an  instrument  that  com- 
bines the  valuable  points  of  the  Landolt  and  Priestly  Smith 
perimeters,  having  the  broad  metal  arc,   the  revolving  chart 


216 


INSTRUMENTS 


No.    160  De  Ztiiu   IViiuu  I.  i.  Xo.   475  De  Zeng  Perimeter. 

holder,  and  the  sight  hole  through  the  center  of  rotation.  It 
has  a  double  adjustable  chin  rest  and  a  clip  for  determining 
the  fixation  point.  The  chart  moves  with  the  rotation  of  the 
arc  and  is  located  just  back  of  a  stationary  scale,  which  is 
graduated  in  red  and  white  to  correspond  with  the  calibrations 
on  the  arc.  No.  460  and  Xo.  475  are  made  by  the  De  Zeng 
Standard  Co..  Camden,  N.  J. 

Other  perimeters  on  the  market  are  De  LaPersonne's,  Uni- 
versal Registering,  Forster's,  Skeel's  Self-Registering,  Hare's 
Automatic,  Dana's  Pocket  Perimeter,  Spiller's  Hand  Peri- 
meter.    See  also  Campimeter. 

Among  the  manufacturers  of  ju'rimeters  is  the  (leneral  Op- 
tical Co.,  Mt.  Vernon,  N.  Y. 

PLACIDOSCOPE. 

Instrument  for  making  an  examination  of  the  cornea,  includ- 
ing both  central  and  peripheral.     Consists  of  a  black  disc  with 


I  'hiiiild.scnpc. 

wliiti-  concentric  circli-s,  with  short  focus  lens  in  center.  Pii- 
niarily  constructed  to  detect  corneal  ;i>ti<;niatisin.  .Made  by  \'\ 
:\.  Hardy  \'  Co.,  (  hicago. 


INSTRUMENTS  217 

PRENTICE   PHORIA    INDICATOR. 

An  instrument  which  is  very  useful  for  indicating  and  meas- 
uring the  amount  of  phorias  existing  in  a  patient's  eye ;  in- 
tended especially  for  determining  hyperphoria,  etc.,  to  be  cor- 
rected by  prisms.     Designed  after  suggestions  by  Charles  F. 


Prentice  Phoria  Indicator. 

Prentice.  Made  in  the  form  of  a  cross,  with  an  electric  lamp, 
which  illuminates  the  eight  vertically  and  eight  horizontally 
arranged  green  glass  discs,  placed  directly  behind  a  small  red 
glass  disc  in  the  center.  Made  hy  the  Bausch  &  Lomb  Optical 
Co.,  Rochester,  N.  Y. 

PUNCTUMETER. 

An  instrument  produced  for  measuring  accommodiation 
quickly  and  accurately.  It  consists  of  a  tube  ten  centimeters 
long,  into  one  end  of  which  is  inserted  a  plus  lens.  Two  cells 
for  holding  trial  lenses  are  attached  to  the  other  end  of  the 
tube,  one  of  them  being  graduated,  enabling  the  operator  to 
place  the  axis  of  cylindrical  lenses  in  the  proper  meridians.  A 
bar  upon  which  is  mounted  a  sliding  target  operated  by  a  rack 
and  pinion  is  attached  to  the  end  of  the  tube  carrying  the  lens. 
This  bar  also  carries  a  supplementary  slide  which  can  be  fixed 


218 


INSTRUMENTS 


Punctumeter. 

at  any  point  on  the  bar  by  a  set  screw.     Made  by  F.  A.  Hardy 
&  Co.,  Chicago. 

SPECTRUM  PROJECTOR. 

This  instrument,  which  is  very  easy  to  use,  gives  a  conven- 
ient and  sensitive  means  of  studying  the  absorbing  effect  of 
various  substances  on  the  visible  and  on  the  ultra  violet 
regions  of  the  spectrum.  It  is  adapted  for  absorption  determin- 
ation of  ophthalmic  lenses.  Furnished  in  two  styles;  mounted 
on  w^ooden  base  or  with  supplementary  base  plate  and  metal 
screens  for  testing  color.  Made  by  the  Bausch  &  Lomb  Op- 
tical Co.,  Rochester.  X.  Y. 

STEREO   CAMPIMETER. 

This  instrument  permits  binocular  fixation  in  perimetry  and 
cami)imetry  examinations.     The  field  of  the  same  extends  10 


,Sti'i-fi>  Campliiiotfr. 


INSTRUMENTS  '  219 

degrees  nasally,  40  degrees  temporally,  30  degrees  above  and 
30  degrees  below  the  center  of  fixation.  The  equipment  con- 
sists of  a  metal  stand  with  adjustable  stereo  lens  attachment, 
Lloyd  Campimeter  Slate  mounted  on  small  metal  support,  100 
Lloyd  Campimeter  Record  Charts,  set  of  test  objects  and 
Wells  Chart.  Made  by  the  Bausch  &  Lomb  Co.,  Rochester, 
N.  Y. 

STEVENSON'S  MUSCLE  TEST. 
Devised  by  Dr.  Mark  D.  Stevenson  to  determine  the  close 
relationship  between  accommodation  and  convergence.     This 


Stevenson's  Muscle  Test. 

test  helps  to  decide  the  best  reading  distance  for  the  patient. 
Also  frequently  important  in  prescribing  prisms  or  the  center- 
ing of  lenses.     Made  by  F.  A.  Hardy  &  Co.,  Chicago. 

TEST  CABINET. 

An  arrangement  for  holding  and  illuminating  test  charts 
used  in  subjective  testing.  Hundreds  of  cabinets,  electrical 
and  otherwise,  and  made  in  various  designs  and  colors  to 
match  the  furniture  of  the  office,  some  being  on  stands,  some 
on  brackets,  others  for  mounting  on  walls.  Made  by  the 
Bausch  &  Lomb  Optical  Co.,  Rochester,  N.  Y. ;  M.  E.  Green 
Mfg.  Co.,  St.  Louis,  Mo. ;  Globe  Optical  Co.,  Boston,  Mass. ; 
F.  A.  Hardy  &  Co.,  Chicago ;  Merry  Optical  Co.,  Kansas  City, 
Mo. ;  Gould  Optical  Equipment  Co.,  New  York,  and  others. 


220  INSTRUMENTS 

TEST  LENSES  (Ophthalmic),  VERTEX. 
These  are  num!)ered  in  vertex  refraction.  Kach  lens  in  the 
set  being  of  the  same  thickness,  a  given  cylinder  always  gives 
the  same  cylindrical  effect,  regardless  of  the  sphere  with  which 
it  may  be  coml)ined.  The  lenses  are  15  mm.  in  diameter, 
mounted  in  32  mm.  discs,  and  give  ap])roximately  axial  refrac- 


I'.ausch  ^.    ),.  nib  <,iiilitlialuiic  'l\^l  Lt-ntes. 


tion.  Can  be  had  in  jxjrtable  case;  also  in  sanitary  glass  and 
metal  enameled  cabinet  having  a  small  formalin  vajxirizer  for 
sterilization  purposes  as  well  as  space  for  the  test  frame  and 
other  accessories ;  it  may  also  be  had  in  white  enameled  cabi- 
net (wood)  with  opal  glass  plates  at  each  end  atTt)r(ling  space 
for  auxiliary  instruments,  drawers  below  for  test  frame,  etc., 
also  four  more  drawers.  Made  by  the  liausch  iv  I.omb  (  >ptical 
Co.,  Rochester,  N.  Y. 

TONOMETER. 
Designed    for   measurin;;    the   tinsinn   of    the   eyeball,   espe- 
cially valuable  in  cases  of  j^laiucm.i.      TIuTe  are  sever.il  types 
of  tonometers. 

TRIAL  CASE. 

Contains   splurical    and   cylindrical     lenses    and     otlu'r    ac- 
cessories used  in  siibjecti\  e  refraction.     Made  by  the  .American 


INSTRUMENTS 


221 


American   Optical   Co.    Trial   Case. 

Optical  Co.,  Southbridge,  Mass.,  and  Bausch  &  Lomb  Optical 
Co.,  Rochester,  N,  Y. 

TRIAL  FRAME. 

Made  with  three  cells,  two  of  which  are  revolved  by  thumb 
screws.     The  bridge  is  self-adjusting  for  angle  of  the   nose. 


American  Optical  Co.   Trial  Frame. 


Frame  is  adjustable  for  pupillary  distance,  height  of  bridge 
and  length  of  temple.  Supplied  either  with  comfort  or  wire 
temples.  There  are  quite  a  variety  of  trial  frames  made  by  the 
same  concern,  including  some  for  presbyopic  correction  only, 


222 


INSTRUMENTS 


as  also  square  cell  frames  for  prisms,  and  many  other  kinds. 
Made  by  The  American  Optical  Co.,  Southbridge,  Mass. 

TRIAL  FRAME   (California). 
It  consists  of  a  specially  constructed  adjustable  and  folding 
head-band — deep  set  eye  lenses  can  be  set  in  close — with  pro- 
truding eyes  or  long  eyelashes  lenses  can  be  set  accordingly. 


A.;_ 


CalifoinKi    I  I 


I'lMllU-. 


The  reading  angle  is  obtained  by  tilling  lens  cells  forward 
The  axis  scale  is  placed  inside  of  lens  cell.  The  frame  can  be 
taken  apart  and  folded  in  a  compact  fdiin.  .Made'  by  the  Ameri- 
can Optical  Co.,  Southbridge,  Mass. 

TRIAL  FRAME  (Gcnothalmic). 

'i'his   frame  possesses  straight    spring  temples,   which   ha\c 

just  enough  tension  to  hug  the  face  comfortably  and  securely. 

The  straight  temi)les  permit   of  easy   jjutting  on   and  olT  the 

frame.    A  tilting  device  allows  the  rntin-  franu-  to  be  tilted  for- 


INSTRUMENTS 


223 


Genothalmic   Trial    Frame. 


ward  for  the  reading  test.    The  temples  remain  in  their  original 

position.    Made  by  the  General  Optical  Co.,  Mt.  Vernon,  N.  Y. 

TRIAL   FRAME    (Genothalmic   Clinical). 

It  consists  essentially  of  two  steel  plates  joined  at  the  center 
by  extension  pieces  which  are  toothed  inside  to  form  a  double 
rack.  An  adjustment  for  interpupillary  distance  is  accom- 
plished by  placing  a  pinion  between  the  racks.  The  bridge  is 
brought  to  the  required  height  by  pushing  the  pinion  up  and 
down  in  the  rack.  The  dials  are  engraved  on  a  stationary 
steel  rack  which  holds  the  lenses.  The  temples  are  the  same 
as  those  in  the  Genothalmic  Frame.  Made  by  the  General 
Optical  Co.,  Mt.  Vernon,  N.  Y. 

TRIAL  FRAME,   PRECISION   (Precision  Test   Frame). 

A  test   frame  scientifically  constructed  throughout,  having 


Precision  Test  Frame. 


224  INSUFFICIENCY 

all  the  adjustments  necessary  to  enable  the  operator  to  make 
accurately  and  (luickly  the  measurements  required.  Its  man- 
ipulation is  simple.  It  is  not  necessary  to  write  details  of  an 
examination  during  the  examination,  as  they  can  be  secured 
from  the  scales  on  the  frame.  Made  by  the  Bausch  &  Lomb 
Optical  Co.,  Rochester,  N.  Y. 

TRIAL  FRAME  (Stoco). 
The  frame  is  constructed  so  that  each  eye  and  each  temple 
move  independently,  enabling  the  operator  to  detect  and 
record  any  deviation  from  symmetry  in  features.  It  has  a 
graduated  bar  at  the  top  carrying  two  sets  of  brackets.  The 
temples  are  operated  independently  of  one  another  by  means  of 
right  and  left  screws.  The  nose  piece  has  a  swinging  bridge 
to  fit  any  shape  of  nose.  The  tcmi)les  slide  and  change  in 
length  by  means  of  a  small  set  screw.  Manufactured  in  two 
sizes  of  eye — lj4  in-  'in<J  l/S  in.  Can  be  had  with  two  or  three 
cells.  There  are  many  other  kinds  u\  trial  frames  manufac- 
tured by  the  same  firm.  Made  by  the  Standard  Optical  Co., 
Geneva,  N.  Y. 

VISUAL  ACUITY  TEST  WITH  IVES  SCREEN. 
This  apparatus  consists  of  parallel  lines  or  of  sc|uares,  the 
distance  between  which  in  the  first  case  or  size  in  the  second 
can  be  varied  continuously  from  invisibility  to  easy  visibility. 
It  also  lends  itself  admirably  to  the  determination  of  the  axis 
of  astigmatism.  Made  by  the  Bausch  (\:  Lomb  Optical  Co., 
Rochester,  N.  Y. 
Insufficiency.  A  term  ap])lic(l  to  the  condition  of  the  nuiscle 
which,  in  strabismus  or  in  imbalance,  appears  to  be  the  weaker. 
Thus,  in  internal  strabismus,  we  speak  of  insutTicienc\'  of  the 
external  muscles. 

Intercilium.     '\'\\v  sp.uH'  ln'twcTU  the  eyebrows. 

Intemus.     Applied,  for  short,  to  thf  inlcrual  rectus  nuiscle. 

Interorbital.     I'etween  the  orbits. 

Interval,  Sturm's.  'I  he  interxal  bctwiiii  the  anterior  and  pt)s- 
terior  foci  in  astigmatism.  Sturm  found  that  distant  objects 
are  seen  in  the  posterior  inter\al  and  near  objects  witli  the 
anterior. 


INTRAOCULAR  225 

Intraocular.    Within  the  eyeball. 

Intraorbital.     Situated  within  the  orbit. 

Intumescent  Cataract.  The  rapid  development  of  a  senile  cata- 
ract, sometimes  due  to  acute  glaucoma. 

Iridalgia.     Pain  in  connection  with  the  iris. 

Iridallochrosis.    Change  of  color  in  the  iris. 

Iridauxesis.     The  same  as  Iridoncus. 

Iridectomy.  The  operation  of  cutting  away  a  small  piece  of  the 
iris.  It  is  done  (a)  for  optical  purposes,  to  improve  vision,  (b) 
to  relieve  intraocular  tension,  and  (c)  as  a  preliminary  to  cata- 
ract extraction. 

Iridenclesis.     An  operation  for  displacing  the  pupil. 

Irideremia.     Defective  iris. 

Iridesis.    Tying  off  a  portion  of  the  iris  to  form  an  artificial  pupil. 

Iridescent  Vision.  An  appearance  of  colors  around  the  borders 
of  the  image. 

Iridic.    Pertaining  to  the  iris. 

Iridoavulsion.    Tearing  of  the  iris  from  its  periphery. 

Iridocele.     Hernia  of  the  iris. 

Irido-Chorioiditis.    Joint  inflammation  of  the  iris  and  choroid. 

Iridocinesis.    Expansion  and  contraction  of  the  iris. 

Irido-Cyclitis.  Joint  inflammation  of  the  iris  and  the  ciliary 
body. 

Iridodesis.    A  form  of  iridoclesis. 

Iridodialysis.    Separation  of  the  iris  from  the  ciliary  body. 

Iridodonesis.    Trembling  of  the  iris. 

Iridoncus.    Swelling  or  tumor  of  the  iris. 

Iridoperiphacitis.     Inflammation  of  the  iris  and  lens  capsule. 

Iridoplania.     Trembling  of  the  iris. 


226  IRIS 

Iridoplegia.     Paralysis  of  the  iris.     See  Reflex. 

Iridorrhexis.     Rui)ture  of  the  iris. 

Iridosclerotomy.     l^uticture  of  the  sclera  and  iris. 

Iridotomy.  The  cutting  of  a  small  slil  in  tlie  iris.  It  is  done  for 
the  same  purposes  as  iridectomy. 

Iris.  The  vascular  curtain  which  hangs  suspendecl  in  the  acpie- 
ous  humor,  in  front  of  the  crystalline  lens.  It  is  really  a 
part  of  the  second  tunic  of  the  eye.  of  which  the  chorioid  and 
the  ciliary  body  are  also  parts.  It  springs,  in  fact,  from  the 
anterior  surface  of  the  ciliary  body,  its  free  margin  extending 
into  the  aqueous  chamber.  It  steadies  itself  against  the  cry- 
stalline lens  at  its  back,  so  that  it  forms  a  shallow  cone, 
corresponding  to  the  anterior  convexity  of  the  lens,  with  its 
apex  looking  forward.  In  the  absence  of  the  lens,  (as  after 
cataract  operation),  the  iris  may  be  seen  to  tremble,  lacking 
its  support.  Similar  trembling,  in  very  slight  degree,  may  be 
seen  when  the  pupil  is  greatly  contracted,  because  the  margin 
of  the  iris  floats  away  from  the  lens  and  the  iris  itself  is  much 
thinned. 

The  vessels  of  the  iris,  derived  from  the  long  posterior  and 
the  anterior  ciliary  arteries,  run  from  the  ciliary  to  the  jnipil- 
lary  margin,  and  near  the  latter  margin  they  interlace  to  form 
the  minor  circle  of  the  iris,  dividing  the  iris  into  two  zones, 
the  ciliary  and  the  pupillary  zone.  On  the  pupillary  margin 
there  is  a  black  fringe,  wdiich  becomes  \  cry  \  isiblc  in  a  catar- 
actous  eye,  because  of  its  contrast  with  tlu-  yellowish-white 
lens.  Around  the  minor  arterial  circle  and  around  the  ciliary 
root  of  the  iris  are  indentations  and  crypts,  devoid  of  mem- 
brane, which  are  really  apertures  leadini;  into  tlu'  interit)r  of 
the  iris  substance.  These  openings  mechanically  fa\or  the 
changes  of  size  which  the  iris  imdergoes  during  expansittn  and 
contractioTi. 

The  stroma  of  the  iris  consists  priiui|)ally  of  radiating  ves- 
sels numing  from  the  ciliary  to  the  pupill.iry  margin.  Near 
the  pui)illary  mar}.;in  is  the  sphincter  musclf  iA  tlu-  iris,  which 
contracts  the  pui)il.  embeddetl  in  liie  stroma.  (  )n  the  anterior 
surface  of  the  iris  are  two  membranes,  \  i/..,  the  anterior  limit- 


IRIS  227 

ing  membrane  and  a  continuation  of  Descemet's  membrane. 
On  the  posterior  surface  there  are  also  two,  viz.,  the  posterior 
limiting  membrane,  consisting  chiefiy  of  the  radiating  muscles 
which  dilate  the  pupil,  and  the  retinal  pigment  layer.  This 
latter  layer  represents  the  continuation  of  the  retina,  and  may 
be  regarded  as  the  retinal  part  of  the  iris,  as  distinguished 
from  the  uveal  part. 

COLORING. 
The  iris  contains  two  pigments :  one  in  the  stroma,  which 
is  brown,  and  one  in  the  retinal  pigment  layer,  which,  seen 
through  the  iris,  appears  blue  or  gray.  The  color  of  the  eye 
is  therefore  either  brown  or  blue,  according  to  the  domination 
of  one  or  other  of  these  two  pigments.  If  the  stroma  pigment 
is  plentiful  the  color  is  brown ;  if  scanty,  it  is  blue.  If  the 
stroma  pigment  is  scanty,  but  the  iris  itself  thick,  the  color  is 
gray.  Children  are  born  always  with  blue  eyes,  but  during 
the  first  few  years  of  life  the  color  changes,  owing  to  the 
thickening  of  the  iris  stroma.  Occasionally  this  change  may 
develop  in  one  e^^e  after  birth  and  not  in  the  other,  in  which 
case  one  eye  is  blue  and  the  other  brown.  Not  infrequently, 
while  the  general  stroma  pigmentation  is  poor,  there  may  be 
isolated  spots  in  which  the  pigment  is  thick  which  gives  a 
flecked  or  mottled  appearance  to  the  eye. 

MUSCULATURE,  .    ,,   ^r 

The  iris  is  furnished  with  two  sets  of  muscles:  (1)  the  con- 
centric muscles,  whose  contraction  diminishes  the  size  of  the 
pupil,  and  whose  nerve  supply  comes  from  the  third  nerve, 
through  the  ciliary  ganglion,  and  (2)  the  radiating  muscles, 
whose  contraction  dilates  the  pupil,  and  whose  nerve  supply 
comes  from  the  cervical  sympathetics.  These  two  sets  of 
muscles  oppose  each  other.  Light  falling  on  the  retina  causes 
the  concentrics  to  contract ;  sensory  impulses  from  anywhere 
in  the  body  cause  the  radiating  muscles  to  contract.  Which- 
ever of  these  two  stimuli  is  strongest  at  the  moment  determines 
the  size  of  the  pupil. 

Optically,  the  function  of  the  iris  is  to  vary  the  size  of  the 
pupil,  so  as  to  serve  as  a  cut-ofT,  to  protect  the  retina  against 
excess  of  light,  and  to  insure  properly  graduated  illumination 
for  a  clear  image.  '  - 


228  IRIS  SHADOW 

Iris  Shadow.  The  shadow  which  the  iris  casts  on  a  cataract. 
As  long  as  this  shadow  is  visible  the  cataract  is  not  yet  ripe. 
When  it  is  ripe,  the  iris  no  longer  casts  a  shadow  on  it. 

Iritic.     Pertaining  to  the  iris. 

Iritic  Angle.  The  angle  formed  l)y  the  junction  of  the  iris  and 
the  cornea. 

Iritis.  InHammation  of  the  iris,  usually  due  to  some  internal 
disease,  such  as  rheumatism,  syphilis,  etc.  The  symptoms  are 
not  difficult  to  recognize:  Pain,  worse  at  night,  because  then 
the  pupil  is  dilated  and  the  iris  bunched  upon  itself;  dimness 
of  vision  due  to  the  outpouring  of  muco-serum  into  the  aque- 
ous humor;  cloudinness  of  the  media,  from  the  same  cause; 
ragged  appearance  of  the  iris  itself,  which  also  loses  its  beau- 
tiful lustre  Occasionally  the  iris  becomes  glued  down  to  the 
lens  by  the  exudates.  The  pain  of  iritis  is  usually  felt  in  the 
brow. 

It  is  a  serious  condition,  and  should  be  referred  at  once  to 
an  oculist. 

Irregular  Astigmatism.    Sec  Astigmatism. 

Ischemia.     Lack  of  blood  in  a  part. 

Ischemia  Retinae.    Pallor  of  the  retina  due  to  bloodlessness. 

Isocoria.     Equality  in  the  size  of  the  pupils. 

Isometropia.  Equality  in  kind  and  degree  in  the  refraction  of 
the  two  eyes. 

Isopter.  A  curve  in  the  visual  field  drawn  through  points  of 
equal  acuity  of  vision. 

Isosceles.  Term  ai)plicd  to  a  triangle  which  has  two  of  its  sides 
equal. 

Jaeger's  Test  Type.     A  type  for  testing  near  vision.     Sec  Type. 

Joffroy's  Symptom.  A  symjjtom  observed  in  exophthalmic 
g(jitre.  W'lien  the  eyes  arc  suddenly  turned  upward,  tlure  is 
no  facial  contraction.    It  is  the  obverse  of  Graefe's  Sign. 

Kataphoria.    Sec  Cataphoria, 


KAYSER'S  DISEASE  229 

Kayser's  Disease.  An  affection  marked  by  a  greenish  discolora- 
tion of  the  cornea — associated  with  different  organic  condi- 
tions. 

Keratalgia.     Pain  in  the  cornea. 

Keratectasia.    Protrusion  of  the  cornea. 

Keratitis.  Inflammation  of  the  cornea.  When  the  disease  is 
acute  there  is  dull  aching  pain  in  the  eye,  injection  of  the 
circum-corneal  vessels,  watering  of  the  eye,  and  steaminess 
of  the  cornea.  When  chronic,  the  cornea  is  dull  and  partially 
opaque,  lessened  in  sensibility,  and  painless. 

The  injection  of  keratitis  is  a  ciliary  injection,  and  is  to  be 
distinguished  from  that  of  conjunctivitis  by  being  pink  in 
color,  located  on  either  side  of  the  cornea,  and  none  of  the 
separate  vessels  being  distinguishable. 

Interstitial  keratitis,  in  which  the  cornea  becomes  white  and 
opaque,  is  usually  due  to  inherited  syphilis. 

Keratocele.  Protrusion  of  Descemet's  membrane  through  the 
outer  layers  of  the  cornea. 

Keratoconus.  A  condition  of  the  cornea  in  which  it  is  bulged 
forward  in  the  form  of  a  cone.  Refraction  is  of  course  inter- 
fered with. 

Keratoglobus.    A  globular  protrusion  of  the  cornea. 

Keratohelcosis.     Ulceration  of  the  cornea. 

Kerato-Iritis.    Joint  inflammation  of  cornea  and  iris. 

Keratomalacia.     Softening  of  the  cornea. 

Keratome.    A  lance-shaped  knife  for  opening  the  cornea. 

Keratometer.    Same  as  an  ophthalmometer  q.  v. 

Keratomycosis.     Fungus  growths  on  the  cornea. 

Keratonyxis.     Rupture  of  the  cornea. 

Keratoplasty.     Plastic  surgery  of  the  cornea. 

Keratcscleritis.  Joint  inflammation  of  the  cornea  and  the 
sclera. 


230  KERATOSCOPE 

Keratoscope.    An  instrument  for  examining  the  cornea. 
Keratoscopy.     Examination  of  the  cornea. 
Kerectomy.     Removal  of  part  of  the  cornea. 
Kilometer.     One  thousand  meters. 
Kopiopia.     See  Copiopa. 

Korectomia.     Making  tjf  an  artificial  pupil   by  reuKjving  a  part 

()\   the  iris. 

Korectopia.     Displacement  of  the  pupil. 

Koroscopy.    Same  as  Retinoscopy. 

Kryptok.  A  trade  name  given  to  a  patented  bifocal  lens,  made 
by  fusing  together  two  pieces  of  glass,  of  differing  density,  for 
the  distance  and  the  near  correction.  resi)ecti\cly.     See  Lens. 

L.  and  L.  E.    Abbre\iation  for  left  eye. 

Lachrymal  Apparatus.  The  apparatus  for  secreting  and  dispos- 
ing of  the  tears.  The  tears  are  secreted  by  the  lacrymal  gland, 
which  consists  of  two  parts,  the  superior,  lying  in  the  upper 
outer  angle  of  the  orbit,  and  the  inferior  (accessory)  lying 
under  the  mucous  memljrane  of  the  fornix.  l»uth  emjUy  into  the 
conjunctival  sac  by  means  of  ducts. 

The  tears  drain  through  the  lacrymal  sac,  which  opens  from 
the  lower  eyelid,  near  the  inner  fornix,  with  the  puncta  lacry- 
malia  ;  thence  through  the  lacrymal  duct  into  the  nasal  cavity, 
just  behind  the  lower  turbinate  bone. 

Lachrymation.     ()\ertlow  of  tears  from  the  eyes. 

Lachrymotony.  ("iittinj^  of  the  laclir\  mal  «hict  to  cure  a  stric- 
ture. 

Lacrimal.    .Same  as  Lachrymal. 

Lacrymal.     The  same  as  Lachrymal. 

Lacuna  Orbitae.      The  roof  of  liie  orbit. 

Lacus  Lachrymalis.  The  area  of  the  eyi-  ne.ir  the  imier  r;inthus 
where   the   tears  Collect. 


LAEVOPHORIA  234 

Laevophoria.  A  conjugate  muscular  imbalance,  in  which  both 
eyes  tend  to  turn  toward  the  left. 

Lagophthalmia.  A  condition  in  which  it  is  impossible  to  close 
the  eyes. 

Lakus.  The  small  circular  opening  at  the  inner  fornix  of  the 
eyelids. 

Lambert's  Law.  The  amount  of  light  which  passes  through  a 
colored  iilter  or  glass  plate  diminishes  in  geometrical  progres- 
sion as  the  thickness  increases  in  arithmetical  progression. 

Lamina  Cribrosa.  Literally,  a  perforated  plate.  Fibrous  bands 
of  the  chorioid  which  bridge  over  the  foramen  sclerae,  pierced 
by  the  funiculi  of  the  optic  nerve.  It  is  the  weakest  spot  in 
the  tunics  of  the  eyeball,  and  therefore  the  first  to  give  way 
under  intra-ocular  pressure,  as  in  glaucoma. 

Lamina  Fusca.    The  outer  layer  of  the  cornea. 

Lamps.    See  Illumination. 

Landolt's  Bodies.  Small  bodies  found  between  the  rods  and 
cones  of  the  retina. 

Lantern  Tests.     Tests  for  color-blindness,  q.  v. 

Lapsus.     Dropping  of  the  upper  lid.     See  Ptosis. 

Lashes.     The  hairs  that  grow  upon  the  margins  of  the  e^-elids. 

Latent.  That  which  is  not  made  manifest,  but  exists  only  po- 
tentially. In  optics  it  is  especially  applied  to  hyperopia  which 
has  taken  the  form  of  a  muscle  spasm,  and  cannot  be  brought 
out  by  tests  (See  Hyperopia),  and  imbalance  of  the  extrinsic 
muscles  which  has  not  yet  become  actual  squint.  (See  Stra- 
bismus). iiO^ 

Lateroduction.     See  Muscles. 

Law.  The  expression  of  a  sequence  of  natural  events  which  oc- 
cur under  a  given  set  of  conditions. 

Angstrom's  Law.  The  wave-lengths  of  the  light  absorbed 
by  a  substance  are  the  same  as  those  given  off  by  it  when 
luminous. 


232  LAW 

Brewster's  Law.  For  any  substance  the  polarizing  angle  is 
equal  to  the  angle  of  incidence  at  which  the  portion  of  light 
that  is  reflected  is  at  right  angles  to  the  portion  that  is  re- 
fracted. 

Descartc's  Law.  The  sine  of  the  angle  of  incidence  bears  a 
constant  ratio  to  the  sine  of  the  angle  of  refraction  for  two 
given  media. 

Donder's  Law.    See  Bonders. 

Giraud-Telon's  Law.  Binocular  retinal  images  are  formed 
at  the  intersection  of  the  primary  and  secondary  axes  of  pro- 
jection. 

Gullstrand's  Law.  If,  when  a  patient  is  made  to  turn  his 
head  while  fixing  a  distant  object,  the  corneal  reflex  from  either 
eye  moves  in  the  direction  in  which  the  head  is  turning,  it 
moves  toward  the  weaker  muscle. 

Kirchoff's  Law.  When  a  pencil  of  light  is  passed  through  a 
transparent  body,  the  latter  absorbs  only  those  luminous  rays 
which  it  is  capable  of  emitting  when  heated  to  incandescence. 
(This,  as  will  be  seen,  is  the  equivalent  of  Angstrom's  law). 

Malus'  Law.  An  orthotomic  system  of  rays  remains  ortho- 
tomic,  no  matter  what  refractions  or  reflections  the  rays  may 
undergo  in  traversing  a  series  of  isotropic  media. 

Law  of  Absorption.  The  proportion  of  transmitted  radia- 
tion varies  geometrically  as  the  thickness  of  the  absorbing 
medium  varies.     See  Absorption. 

Law  of  Projection.  An  impression  made  on  any  point  of  the 
retina  is  projected  outward  into  the  visual  field  following  the 
line  of  direction,  i.  e.  a  line  passing  from  that  retinal  point 
through  the  nodal  point  of  the  eye. 

Law  of  Reflection.     See  Reflection. 

Law  of  Refraction.    See  Refraction. 

Law  of  Sines.  The  sine  of  the  angle  of  incidence  is  equal 
to  the  sine  of  the  angle  of  rellection  multii)lied  by  a  constant. 

Prentice's  Law.  Any  lens  is  capable  of  producing  as  many 
prism  dioptres  as  the  lens  possesses  (li(»])trrs  of  refraction,  pro- 
vided it  is  decentered  one  centimeter. 

Talbot's  Law.  When  complete  fusion  occurs,  aiul  the  sensa- 
tion is  uniform,  the  intensity  is  the  same  as  would  occur  were 
the  same  anioimt  of  light  spread  uniformly  over  the  disc. 


LENS  233 

Wundt-Lamansky's  Law.  The  line  of  vision,  in  moving 
through  a  vertical  plane  parallel  to  the  frontal  plane,  moves  in 
straight  lines  in  the  vertical  and  horizontal  directions,  but  in 
curved  paths  in  all  other  movements. 

L.  D.  Abbreviation  for  light  difference,  denoting  the  different 
light  reaction  of  the  two  eyes. 

Leber's  Disease.    Hereditary  optic  atrophy. 

Lema.     Dry,  hard,  yellow  incrustation  in  the  inner  canthus. 

Lens.  In  general,  a  lens  may  be  defined  as  any  agent  which 
transmits  and  refracts  light  waves,  in  such  a  way  as  to  produce 
an  image  of  the  object.  It  has  three  principal  functions:  (1) 
to  produce  an  image  larger  than  the  object,  as  in  microscopes ; 
(2)  to  produce  an  image  smaller  than  the  object,  as  in  photo- 
graphic camera:  and  (3)  to  produce  an  image  at  a  desired  focal 
plane,  as  in  ophthalmic  lenses. 

As  the  curvature  of  a  light  wave  is  spherical,  and  as  it  is  the 
purpose  of  ophthalmic  lenses  to  bring  light  waves  to  a  series 
of  focal  points  similar  to  those  from  which  they  originated,  it 
follows  that  the  curvature  of  ophthalmic  lenses  must  be  simi- 
lar in  nature  to  that  of  the  waves,  i.  e.  spherical.  Frequently, 
however,  it  is  desired  to  influence  the  light  wave  in  only  half 
of  its  meridianal  curve,  and  for  this  purpose  lenses  are  em- 
ployed whose  curvature  is  a  segment  of  a  cylinder,  which  may 
be  regarded,  in  its  optical  effect,  as  the  split  half  of  a  sphere. 
Ophthalmic  lenses,  then,  are  spherical  and  cylindrical ;  com- 
monly known  as  spheres  and  cylinders. 

In  accordance  with  the  laws  of  refraction,  light  waves,  upon 
entering  glass  lenses,  whose  refractive  index  is  greater  than 
that  of  air,  are  refracted  toward  the  normal.  Hence,  in  order 
to  render  a  light  wave  more  convergent  it  is  necessary  that  the 
surface  of  the  lens  be  convex ;  to  render  it  less  convergent,  or 
more  divergent,  it  must  be  concave.  Ophthalmic  lenses, 
therefore,  are  convex  and  concave ;  commonly  termed  plus 
and  minus,  respectively. 

Images  of  objects  are  made  by  lenses  in  accordance  with  a 
theory  of  optical  representation  first  formulated  by  Abbe.    If 


234  LENS 

a  pencil  of  rays,  i.  e.  the  totality  of  rays  or  waves  proceeding 
from  a  luminous  point  fall  on  a  lens  or  lens  system,  a  section 
of  the  pencil  will  be  transmitted.  The  emergent  waves  will 
have  directions  differing  from  those  of  the  incident  waves; 
the  alteration,  however,  being  such  that  the  transmitted  waves 
are  convergent  in  the  "image-points"  just  as  the  incident  waves 
are  convergent  in  the  "object-points."  With  each  incident 
wave  is  associated  an  emergent  wave ;  such  pairs  are  termed 
"conjugate  waves."  Similarly,  the  object-point  and  the  image- 
point  are  conjugate  points.  All  object-points  lie  in  the  object- 
space,  and  all  image-points  in  the  image-space. 
A  lens  has  four  cardinal  points,  as  follows : 

(1)  An  anterior,  or  first,  principal  focus 

(2)  A  posterior,  or  second,  principal  focus 

(3)  An  anterior  principal  point 

(4)  A  posterior  principal  point     (See  Points) 

.  From  these  four  points  all  object-image  relaticms  can  be  cal- 
culated. 


Ai 


C.d.xut     ^-(3    B<..Cui*'    (.• 


To  determine  the  image-point  for  any  given  ol)ject-point, 
we  first  choose  a  ray  from  the  object-point  which  runs  parallel 
to  the  axis,  and  follow  its  course  through  the  lens  to  the  point 
on  the  further  side  of  the  lens  where  it  nucts  the  a.xis.  namely, 
the  posterior  principal  focus.  We  then  choose  another  ray 
from  the  object-point  which  jjasscs  through  the  anterior  prin- 
cipal focus  and  fciilow  its  course  through  the  lens  (which 
renders  it  parallel  with  tlic  :ixis)  to  the  jxtint  where  it  inter- 
sects the  first  refracted  ray,  projected  throuj^h  the  i)osterior 
princijjal  focus  to  meet  il.  hi  otlur  words,  the  image-point 
will  be  found  at  the  intersection  of  two  rays  from  the  object- 
point,  t>ne  of  which  passes  through  the  anterior  principal  focus 


LENS  235 

and  the  other  through  the  posterior  focus.  This  is  evident, 
because  any  rays  passing  through  these  two  focal  points  are 
conjugate. 

The  above  construction  does  not  apply  when  the  object- 
point  is  at  infinity.  In  that  case  the  image-point  will  be  in 
the  perpendicular  plane  through  the  posterior  principal  focus, 


The  distance  between  the  principal  point  and  the  principal 
focus  on  either  side  of  the  lens  is  called  the  focal  length  of  the 
lens.  The  relative  sizes  of  object  and  image  are  as  their  re- 
spective distances  from  the  anterior  and  posterior  principal 
points. 

In  addition  to  the  four  cardinal  points  mentioned,  a  lens 
has  two  nodal  points — points  within  the  lens  on  the  principal 
axis  from  which  object  and  image  appear  under  the  same 
angle — and  an  optical  centre,  such  that  for  any  ray  which 
passes  through  it  the  incident  and  emergent  rays  are  parallel. 

In  ordinarily  thin  ophthalmic  lenses,  the  principal  points, 
the  nodal  points,  and  the  optical  centre  are  all  so  close  together 
that  they  are  regarded  as  being  identical,  the  lens  is  consid- 
ered as  having  no  thickness,  and  all  focal  measurements  are 
pivoted  on  the  optical  centre.  The  two  refracting  surfaces, 
of  which  every  lens  is  composed,  are  regarded  as  a  single  re- 
fracting surface,  equal  in  dioptric  value  to  the  sum  of  the  two, 
acting  in  the  plane  of  the  optical  centre. 

The  refracting  power  of  such  thin  lenses,  known  as  the 
dioptrism,  depends  upon  two  factors:  (1)  the  degree  or  radius 
of  curvature  of  the  surfaces,  and  (2)  the  index  of  refraction  of 
the  substance  of  which  the  lens  is  made.  The  unit  of  curvature 
is  the  meter  curve,  i.  e.  a  curve  whose  radius  is  1  meter.  The 
unit  of  refracting  power  is  the  dioptre,  i.  e.  the  power  to  focus 
a  neutral  wave  in  1  meter  distance.  The  ratio  between  the 
meter  curve  and   the  dioptre  is  determined  by   the   index  of 


236 


LENS 


refraction.  The  dioptric  power  is  that  percentage  of  the  meter 
curve  denoted  by  excess  index  over  air  (expressed  as  n-1). 
The  formula  for  this  ratio  is: 

D  =  mc  (n-1) 
Or,  if  the  curvature  is  e  xpressed  in  terms  of  the  radius,  since 
this  radius  is  the  reciprocal  of  the  curve,  the  formula  will  be: 
D  =  (n-l)/r 


JiCo^tristn, 


It  must  not  be  supposed,  however,  that  the  index  element 
operates  alike  at  both  surfaces,  i.  e.  that  the  excess  index  of  the 
glass  multiplied  by  the  meter  curve  represents  the  dioptrism 
at  each  surface.  Considering  the  lens,  surface  by  surface,  it 
must  be  remembered  that  the  index-ratio  is  not  the  same  at 
both  surfaces.  When  the  light  enters  the  first  surface,  the 
glass  is  the  refracting  medium  ;  the  index-gain  in  thus  passing 
from  air  to  glass  is  .52;  and  it  is  this  proportion  of  the  glass- 
index  which  operates  at  the  first  surface.  The  dioptrism  of 
the  first  surface,  thcrcfort'.  is  expressed  in  tlu-  follDwiiig 
formula : 

1  (n-1)^    (n-1) 

r,  n  r,n 

On  emergence  frum  the  k-ns  at  the  second  surface,  the  air 
becomes  the  refracting  medium;  the  inde.x-loss  is  .52;  and  it  is 
this  prop(jrtion  of  the  air-index  that  is  operati\e  at  the  second 


LENS  237 

surface.  Furthermore,  the  metric  curve  of  the  second  surface 
is  now  modified  by  the  meter  curve  of  the  light  in  the  lens. 
In  a  bi-convex  lens  both  these  are  minus  curves,  and  the  index 
element,  therefore,  operates  on  the  net  sum  of  the  two  curves. 
Thus,  the  action  at  the  second  surface,  is  expressed  in  the 
formula : 


D«  = 


1  (n-1) 


rz  r,n 


X 


(n-1) 


1 


Thus,  if  we  have  a  bi-convex  lens  whose  front  surface  has  a 
radius  of  curvature  of  16.6  cm.,  and  its  back  surface  a  radius  of 
25  cm.,  and  whose  index  of  refraction  is  1.52  (to  simplify  the 
problem  we  will  call  it  1.50),  then  the  action  at  the  first  surface 
will  be 

.50 

=  2D. 

16.6  X  1.50 
and  the  action  at  the  second  surface  will  be: 
1  .50 

h2X =  3D. 

.25  1.00 

making  the  combined  dioptrism  of  the  two  surfaces  5  D.,  or 
just  .50  of  their  combined  metric  curve. 

In  thick  lenses,  where  the  distance  between  the  two  sur- 
faces is  a  modifying  factor,  this  method  of  calculation  will  not 
apply.  In  such  cases  the  focal  values  of  the  two  surfaces  are 
separated  by  a  distance  (within  the  lens,  on  the  axis)  depend- 
ing upon  the  thickness  of  the  lens,  its  refractive  index,  and  the 
radii  of  curvature  of  the  two  surfaces,  thus  making  the  dis- 
tance of  the  posterior  principal  focus  further  from  the  anterior 
surface  than  the  reciprocal  of  the  combined  dioptrism  of  the 
surfaces,  and  the  distance  from  the  posterior  surface  (the 
vertex)  less.  The  refraction  performed  at  the  posterior  sur- 
face is  technically  known  as  vertex  refraction,  and  the  sub- 
ject will  be  found  discussed  under  that  heading. 

To  determine  the  vertex  refraction  of  a  lens,  if  D  stands  for 
the  required  power,  r^  for  the  radius  of  the  first  surface,  r.^  for 
the  radius  of  the  second  surface,  d  for  the  thickness  of  the  lens, 
and  n  for  the  index,  then, 


238  LENS 


1                  (n-1) 
D,  = + 


(n-1) 
The  distance  of  an  object  from  the  optical  centre  (or.  if 
thickness  is  to  be  considered,  from  the  first  principal  point) 
of  a  lens  is  called  the  anterior  focal  distance  ;  that  of  the  image 
from  the  oi)tical  centre  or  the  posterior  principal  point,  the 
posterior  focal  distance.  Representing  the  former  Ijy  u.  the 
latter  by  v,  and  the  focal  length  of  the  lens  by  f.  then.    . 

Ill 

u         %•         f 
and  this  formula  furnishes  the  basis  for  all  calculations  con- 
nected with  conjugate  relations. 

LENSES  IN  SERIES. 
When  two  or  more  lenses  are  placed  one  behind  the  other, 
on  the  same  principal  axis,  they  are  said  to  be  in  couplets  or 
series.  When  two  such  lenses  are  in  close  apposition,  their 
combined  jxAver  is  practically  the  sum  of  their  indi\idual 
powers — not  exactly,  because  of  the  impossibility  of  exact 
apposition,  but  this  slight  discrepancy  may  be  ignored.  When, 
however,  they  are  separated  by  an  appreciable  space,  the  sep- 
aration modifies  their  joint  power  in  a  way  depending  upon 
the  nature  of  the  lenses.  Negative  power  is  increased,  positive 
powier  decreased,  by  such  separation. 


'l"lu-re  is  a  unit  space  between  any  two  lenses  which  modiru'S 
their  joint  power  by  just  1  I).,  and  this  unit  space  is  the  prod- 
uct of  their  focal  lengths,  'riius,  if  we  take  two  i)lus  lenses  of 
4  1).  and  lU  I),  respectively,  the  focal  lengths  of  tiiese  two 
lenses  are  25  cm.  and  10  cm.  respectively,  antl  the  product  of 


LENS  239 

.2o  and  .10  is  .23.     Separation  of  the  lenses  in  question  by  .25 
cm.  will  reduce  their  joint  power  from  14  D.  to  13  D. 

To  determine  the  eflfect  of  separation  we  subtract  from  the 
sum  of  the  two  individual  lens  powers  the  product  of  these 
powers  and  the  distance  of  separation  as  a  fraction  of  a  meter. 
If  D  stands  for  the  net  power  sought,  D^  for  the  power  of  the 
first  lens,  D,  for  the  power  of  the  second  lens,  and  d  for  the 
distance  of  separation,  then, 

D  =  D,  -f  D,—  (D,  D,  d) 

Thus,  in  the  illustration  just  given,  i.  e.  a  plus  4  D.  and  a 
plus  10  D.  separated  by  2.5  cm.,  we  have 

D  =  4  -f  10  —  (4  X  10  X  .025)  =  13  D. 

Or,  if  we  wish  to  find  the  focal  length  of  the  two  separated 

powers,  we  have  only  to  divide  the  cjuantities  into  unity,  so 

that  the  formula  becomes : 

1 
f  = 

Di  +  D,  —  (Di  Do  d) 

Or,  if  we  are  working  with  the  radii  of  the  lenses  instead  of 

dioptric  values,  then  we  have 

(n-1)  (n-1)         (n-iyd 

D= + 

Ti  r^  ri  r, 

LENSES  WITH  THICKNESS. 

When  the  thickness  of  a  lens  is  to  be  taken  into  account  in 
calculating  its  dioptrism,  its  two  surfaces  really  constitute  a 
couplet  of  separated  lenses  in  which  the  distance  of  separation 
is  traversed  by  the  light  in  glass.  The  same  method  and 
formulae  of  calculation,  therefore,  apply  as  were  applied  to 
separated  lenses,  except  that  the  distance  of  separation,  i.  e. 
the  thickness  of  the  lens  along  its  axis,  must  be  divided  by  the 
index  of  the  glass  in  order  to  reduce  it  to  an  equivalent  of 
thickness  in  air.  That  is  to  say,  the  quantity  represented  by 
thickness  being  expressed,  as  the  distance  of  separation  was, 
by  the  letter  d,  then  d  =  t/n. 

Thus,  if  we  have  a  meniscus  lens,  whose  front  surface  (Dj) 
is  a  plus  6,  its  back  surface  (Do)  a  minus  4,  its  thickness  5  mm., 
and  its  index  1.52,  then  to  calculate  its  net  dioptrism  the  follow- 
ing formula  applies : 


240 


LENS 


D  =  D, +  D,— (DiD,t/n) 
Substituting  the  values  for  the  letters,  we  have 
D  =  6  +  — 4—  (6X— 4X  5/1.52) 
=     2  —  (—24  X  .003283) 
=     2  —  (—.078792) 
=    2  +  .078792  =  2.078792  D. 
which  is  the  true  net  value  of  the  lens. 


The  optical  centre  of  the  lens  is  calculated  as  follows 

0=  D,  t 

anterior  to  back  surface 


6  X  .005 


2 


=  .015  ant.  to  D.. 


The  posterior  principal  point  differs  from  the  above  in  that 
the  divisor  is  the  net  value  of  the  lens  instead  of  its  nominal 
value.    Hence  the  formula  : 
Dit/n 
hj  = forward  of  D^ 


D 

6  X  .003283 


anterior  to  D., 


2.078792 
The  position  of  the  anterior  principal  point  is  similarly  cal- 
culated, except  that  — 4  takes  the  place  of  +6,  and  the  distance 
is  measured  from   the  anterior  surface  or  pole.     This  locates 
it  as  follows: 

—4  X  .003283 

h  = posterior  to   D, 

2.078792 

However,  simc  tlic  factor  — 4  here  is  a  no^^ative  factor,  the 
:«*sult  will  be  a  minus  (listancc,  and  minus-posterior  is  an- 
terior; hence  both  priiicii)al  points  are  in  this  case  anterior  to 
the  front  surface. 


LENS  241 


Ordinary  lenses  are  made  of  crown  glass,  having  a  refrac- 
tive index  of  approximately  1.52.  With  regard  to  their  two 
surfaces,  they  are  made  in  several  different  forms,  as  follows : 

Bi-convex,  in  which  the  action  of  both  surfaces  is  positive. 
The  refractive  effect  is  then  the  sum  of  the  two  surfaces,  both 
surfaces  rendering  light  waves  more  convergent. 

Bi-concave,  in  which  both  surfaces  have  a  negative  action. 
Both  surfaces  then  render  light  waves  less  convergent,  or  more 
divergent,  the  net  result  being  the  sum  of  the  two  actions. 

Convexo-concave  (meniscus),  where  the  action  of  the  front 
surface  is  positive  and  that  of  the  back  surface  negative,  the 
positive  action  dominating,  so  that  the  net  result  is  a  positive 
dioptrism  equal  to  the  difference  between  the  two  surfaces. 

Concavo-convex  (meniscus),  similar  to  the  foregoing,  but 
with  the  negative  surface  dominating,  so  that  the  net  elTect 
is  a  negative  dioptrism  equal  to  the  difference  between  the 
two. 

Plano-convex  and  piano-concaves  are  no  longer  made,  ex- 
cept when  a  prism  is  to  be  ground  on  one  side  of  the  lens. 

Meniscus  lenses  are  the  commonest  form  of  spherical  lens ; 
they  are  also  known  as  periscopic  lenses,  because  they  conform 
to  the  rotation  of  the  eyeball,  permitting  the  visual  axis  to 
cut  the  two  surfaces  of  the  lens  in  any  direction  almost  pen- 
pendicularly. 

Frequently  it  is  necessary  to  make  a  lens  w'hich  shall  have 
both  a  spherical  and  a  cylindrical  effect,  in  which  case  the 


242  LENS 

sphere  is  ground  on  one  surface  and  the  cylinder  on  the  other. 
Such  lenses  are  known  as  compound  lenses. 

Compound  lenses  are  made  either  in  bi-convex  or  bi-concave 
form,  as  the  case  may  be,  or  else  in  meniscus  form,  the  latter 
being  known  as  toric  lenses,  (from  the  Greek  word  tores, 
meaning  the  elliptical  base  of  a  column),  because  the  surface 
on  which  the  cylinder  is  ground  has  an  elliptical  curve.  Toric 
lenses  are  ground  on  standard  spheiical  basei>,  3D,  61).  and 
9D,  according  to  the  required  dioptrism,  the  cylindrical  sur- 
face being  then  ground  so  as  to  give  the  required  compound 
effect.  (Other  bases,  of  different  values,  are  nowadays  used 
by  various  manufacturers).  The  toric  lens  is  by  far  the  best 
form  for  a  compound,  because  it  is  periscopic,  and  reduces  to 
a  minimum  the  prismatic  elTect  which  would  otherwise  be 
experienced  in  looking  obliquely  through  the  sphere  and  cylin- 
der. 

When  it  is  desired  to  combine  a  prism  with  a  lens  this  may 
be  done  either  by  grinding  a  prism  on  one  side  of  the  lens,  or, 
better,  if  there  is  sufficient  dioj:)tric  power  in  the  lens,  by 
decentering  it  sufficiently  to  obtain  the  desired  prism  effect. 
(See  Decentration). 

When  it  is  desired,  in  the  case  of  a  presjjyope,  to  combine  in 
the  same  lens  correction  for  distance  and  correction  for  near 
point,  this  is  accomplished  by  means  of  a  bifocal  lens.  (See 
Bifocal). 

A  spherical  lens  may  be  said  lo  be  a  radiating  scries  of 
prisms,  whose  bases  meet  at  the  ()])tical  centre  of  the  lens  in 
the  case  of  a  convex  lens, — the  ajjices  in  the  case  of  a  concave 
lens.  Hence,  if  we  view  an  object  through  a  spherical  lens 
any  other  than  through  its  ()])tical  centre  there  is  an  apparent 
displacement  of  the  image  in  the  direction  of  the  apices  of 
these  prisms — in  con\L'x  lenses  toward  tin-  periphery  of  the 
lens,  in  concave  lenses  toward  the  centre.  This  displacement 
increases,  the  nearer  to  the  apex  our  \  isual  axis  jiierccs  the 
lens.  The  cfTcct  of  this  is  that  it  w c  iiioxc  a  K-iis  to  and  fro 
between  our  eye  and  an  oliji-i  t,  the  iina^e  will  appear  to  nu»\e 
— in  the  same  direction  lliat  \\i'  nioxc  a  eouia\e  lens,  in  the 
opposite  du'cctioii  to  that  in  which  we  nio\  e  a  convex.  This 
is  what  is  called  the  parallax  of  tlu-  lens;  it  serves  to  identify 


LENS  CAPSULE  243 

the  nature  of  the  lens,,  and  by  finding  a  lens  of  opposite  kind 
which,  when  imposed  upon  it,  stops  this  parallax  movement, 
we  can  determine  its  dioptric  strength.  (See  Neutralization). 
The  same  thing  holds  good  with  a  cylinder  in  the  meridian  of 
its  power,  i.  e.  across  its  axis. 

If  we  look  through  a  cylinder  at  a  straight  line,  other  than 
in  alignment  with  the  axis  of  the  cylinder,  or  at  right  angles 
to  it,  the  continuity  of  the  line  appears  to  be  broken  where  it 
enters  the  range  of  the  cylinder.  By  rotating  the  cylinder,  as 
we  look  through  it,  until  the  straight  line  passes  through 
unbroken,  "vve  know  that  this  straight  line  now  represents 
either  the  axis  of  the  cylinder  or  its  perpendicular.  If  the 
unbroken  line  be  running  along  the  axis  of  the  cylinder,  the 
refractive  power  of  the  cylinder  will  afifect  the  thickness  of  the 
line,  making  it  appear  thinner  in  the  case  of  a  convex  cylinder, 
thicker  in  the  case  of  a  concave,  than  where  it  is  outside  the 
range  of  the  lens.  If  the  unbroken  line  lie  across  the  axis  of  the 
cylinder,  no  change  will  appear  in  its  thickness.  We  are 
therefore  able  to  find  the  axis  of  a  cylinder  by  this  means. 

If,  then,  we  hold  a  cylindrical  lens  upright,  i.  e.  the  way  it 
is  going  to  sei  before  the  patient's  eye,  and  view  through  it  an 
astigmatic  wheel  chart,  we  can  see  at  once  where  the  axis  of  the 
cylinder  lies,  by  noting  the  meridianal  line  of  the  chart  which 
passes  unbroken  through  the  lens,  but  is  changed  in  thickness. 

Lenses,  as  put  up  in  a  trial  case  for  use,  are  numbered  either 
according  to  their  focal  length  or  according  to  their  dioptric 
power ;  sometimes  according  to  both. 

Lens  Capsule.     The  capsule  which  surrounds  and  contains  the 
crystalline  lens.     See  Lens,  Crystalline. 

Lens,  Crystalline.  The  lentel-shaped,  transparent  body  which 
lies  between  the  aqueous  and  vitreous  humcH-s  of  the  eye,  sus- 
pended all  around  by  a  circular  ligament  called  the  zonula 
ciliaris,  formed  by  a  reflection  of  the  hyaloid  membrane.  The 
substance  of  the  lens  is  arranged  in  layers,  and  consists  of  a 
nucleus  and  a  cortical  portion,  enclosed  in  a  fine  capsule.  In 
a  young  healthy  person,  lens  and  capsule  are  both  exceedingly 
elastic ;  but  this  elasticity  gradually  diminishes  during  life, 
.  until,  at  about  seventy  years  of  age,  it  is  lost  altogether  and  the 
lens  becomes  rigid. 


244 


LENS  MEASURE 


The  lens  has  no  blood  supply,  depending  for  nourishment 
upon  the  vessels  at  its  periphery.  Therefore,  under  certain 
conditions  of  internal  disease  and  malnutrition,  it  easily  be- 
comes degenerated  and  opaque.     This  constitutes  cataract. 

Optically,  the  crystalline  lens  is  a  biconvex  lens,  whose 
front  curvature,  at  rest,  has  a  radius  of  10  mm.  and  its  posterior 
surface  a  radius  of  6  mm.,  the  latter  having  the  greater  curva- 
ture. Its  index  of  refraction  at  the  nucleus  is  1.43 ;  at  the  cortex 
it  is  less.  Of  itself,  the  lens  has  a  dioptrism  of  19  or  20  D.,  but 
in  series  with  the  rest  of  the  refracting  media  of  the  eye  this 
power  is  diminished.  A  plus  10  or  11  D.  lens  before  the  eye 
usually  compensates  for  the  loss  of  the  crystalline  lens. 

The  function  of  the  lens,  of  course,  is  to  help  focus  light  upon 
the  retina.  In  accommodation,  the  front  surface  is  made  more 
convex  by  contraction  of  the  ciliary  muscle.  This  faculty 
gradually  diminishes  as  life  advances,  until  it  is  finally  lost. 

Lens  Measure.  Since  the  index  of  refraction  of  crown  glass 
lenses  is  uniform,  the  variant  factor  is  the  curvature  of  the 
lens,  and  any  mechanical  device  for  measuring  this  curvature 


Oclievii    l.cti.s    Aliasiiri 


will  dt'terniiiie  the  dioptric  power  of  the  U-ns.     Such  a  device 
is  accomplished   by   means   of   two    immovable    pins    and    a 


LENS  SIZER  245 

movable  central  pin,  the  two  former  being  held  firmly  against 
the  lens,  the  latter  pressed  up  in  proportion  to  its  convexity 
or  allowed  to  project  in  proportion  to  its  concavity.  It  is 
necessary,  of  course,  to  measure  both  sides  of  the  lens,  and  to 
calculate  their  net  result. 

Lens  Sizer.  An  instrument  for  measuring  the  size  of  the  lens. 
Manufactured  by  the  Merry  Optical  Company. 

Lenses — Nomenclature — Biconvex,  Biconcave,  Concavo,  Convex, 
Concave,  Spherical,  Convex  Spherical,  Cylindrical,  Deltar, 
Korrectal,  Aleniscus,  Piano  Convex,  Periscopic,  Peritoric, 
Prism,  Punktal,  Toric. 

Lenses — Nomenclature  —  (Bifocals)  —  Bisight,  Bitex,  Cement, 
Duplex,  Genothalmic,  Kryptok,  Opifex,  Perfection,  Revelation, 
Ultex,  W^ellsworth  Forty-Five  and  others. 

Lenticonus.     Cone-shaped  curvature  of  the  crystalline  lens. 

Lenticular.     See  lenses. 

Lenticular.     Pertaining  to  the  crystalline  lens. 

Lenticular  Astigmatism.  Astigmatism  due  to  irregularity  in  the 
curvature  of  the  crystalline  lens.     (See  Astigmatism). 

Lentitis.     Inflammation  of  th^  crystalline  lens. 

Leucoma.    Leukoma.    A  white  opacity  on  the  cornea. 

Levator  Palpebrae.     See  Muscles  of  the  Eye. 

Levoduction.     Rotation  of  one  or  both  eyes  to  the  left. 

Ligament.  A  band  of  connective  tissue  which  serves  as  an 
attachment  or  support.     In  the  eye  are  the  following: 

Ciliary  Ligament.  Connects  the  cornea  and  sclerotic,  at 
their  line  of  junction,  with  the  iris  and  external  layer  of  the 
choroid. 

Palpebral  Ligament.  Connects  the  cartilage  of  the  lids  to 
the  orbit. 

Suspensory  Ligament,  or  Ligament  of  Zinn.  A  circular 
ligament  attached  to  the  optic  foramen,  from  which  arise  the 
four  rectus  muscles  and  the  superior  oblique. 


246  LIGHT 

Light.  This  word  has  two  applications.  Olijcctively,  light  is  a 
form  of  physical  energy  which,  when  it  falls  upon  the  retina 
and  stimulates  it,  produces  in  the  brain  the  sensation  of  vision. 
Subjectively,  the  word  is  applied  to  the  sensation  itself.  This 
sensation  manifests  itself  in  two  phenomena,  (a)  visibility,  and 
(b)  color,  both  of  which  will  be  found  discussed  under  their 
respective  headings.  This  article  w^ill  deal  wholly  with  the 
physical  aspect  of  the  subject. 

NATURE  OF  LIGHT. 

The  precise  nature  of  light  is  even  yet  unknown.  .Up  to 
the  beginning  of  the  nineteenth  century  two  theories  of  its 
mode  divided  the  field,  the  corpuscular  theory  of  Newton,  and 
the  wave  theory  of  Christiaan  Huygens ;  and  although  Huy- 
gens'  theory  (1690)  preceded  that  of  Newton  (1704),  the  tre- 
mendous weight  of  Newton's  authority  and  influence  caused 
his  theory  to  be  generally  accepted,  in  i)referencc  to  Huygens', 
for  many  years. 

Newton  taught  that  luminous  bodies  emitted  minute  particles 
of  matter,  which  passed  freely  through  transparent  substances, 
and  whose  impact  on  the  retina  produced  sensation  of  light. 
He  assumed  these  corpuscles  to  be  subject  to  the  same  in- 
fluences of  attraction  and  rcpulsicjn  that  he  hail  ascribed  to 
matter  in  general,  and  to  obey  the  same  laws  of  motion.  Thus 
in  the  interior  of  a  homogeneous  body,  according  to  the  first 
law  of  motion,  a  light  cori)UScle  was  held  to  move  in  a  straight 
line,  because  it  was  acted  upon  L'(|ually  upon  all  siiles ;  but  it 
changed  its  course  at  the  boundary  of  two  bodies,  because  in 
the  thin  layer  near  the  surface  there  was  a  resultant  force  in 
the  direction  of  the  normal.  Light  was  reflected,  according  to 
the  well-known  laws  of  rellection,  if  the  corpuscles  were 
acted  u])on  b}'  a  sufficiently  large  force  nu)\ing  toward  the 
first  medium  ;  otherwise  it  was  refracted. 

Huygens  was  the  first  to  slujw  that  the  explanation  of  opti- 
cal phenomena  can  be  made  to  (lei)en(l  upon  the  wave-surface, 
both  in  isotro])ic  bodies,  in  which  the  surface  of  the  \\a\f  is 
spherical,  and  also  in  crystals  of  double  refraction.  lie 
deduced  the  fanicjus  liu\gens  iirinciple.  which  is  the  foumia- 
tion  of  our  modern  concept  of  light,  namely,  that  "any  point 
that  is  refilled  by   ;i   w;i\e  of   lii^lit    beconies   ;i    new    centre  of 


LIGHT  247 

radiation    from    which    the    disturbance    is    propagated    in    all 
directions."' 

The  overthrow  of  Newton's  corpuscular  theory  was  con- 
tributed to  by  several  inductive  experiments  in  the  early  part 
of  the  nineteenth  century,  but  its  final  doom  was  sealed  by  Fou- 
cault,  who  showed  that  the  velocity  of  light  was  less  in  water 
than  in  air,  which  was  in  opposition  to  the  corpuscular  theory 
and  in  harmony  with  the  wave  theory.  It  must  not  be  for- 
gotten, however,  that,  in  spite  of  the  unsoundness  of  New- 
ton's theory,  he  worked  out  some  exceedingly  valuable  calcu- 
lations regarding  light,  which  are  still  valid. 

PROPERTIES  OF  LIGHT. 

Light  has  two  important  properties,  (1)  Intensity,  and  (2) 
Velocity.  Intensity  represents  the  amount  of  light  packed 
into  a  given  area,  and-,  as  between  different  sources  of  illumin- 
ation, at  a  constant  distance,  depends  upon  the  amount  of 
energy  represented  in  the  light-production.  '  For  the  same 
source  of  illumination,  intensity  varies  with  the  distance  from 
the  point  of  origin ;  since,  according  to  Huygens,  the  light 
wave  is  propagated  equally  in  all  directions  (spherically),  the 
ratio  between  light-activity  and  area  undergoes  a  geometrical 
progression,  so  that  intensity  varies  inversely  as  the  square  of 
the  distance  from  the  point  of  origin ;  which  is  the  same  as 
saying  that  it  varies  inversely  as  the  square  of  the  radius  of 
the  light-wave. 

The  velocity  of  light  is  a  constant,  so  far  as  light  itself  is 
concerned,  and  varies  only  according  to  the  density  of  the 
medium  through  which  it  travels.  Propagated  through  lumin- 
ous ether,  its  velocity  is  about  186,000  miles  per  second.  Other 
substances  offer  varying  degrees  of  resistance  to  its  passage, 
and  its  velocity  is  increased  or  diminished,  as  the  case  may  be, 
as  it  passes  from  one  of  these  mediums  into  another.  These 
changes  in  velocity  bring  about,  under  certain  circumstances, 
a  change  in  the  curvature  of  the  wave  and  the  direction  of  its 
propagation,  which  constitutes  the  phenomenon  of  refraction. 
The  difference  in  velocity  is  expressed  in  the  ratio  between  the 
sines  of  the  angles  of  incidence  and  refraction,  respectively, 
plus  or  minus,  as  the  case  may  be,  as  the  light  passes  from  one 
medium  into  another. 


248  LIGHT 

PHENOMENA  OF  LIGHT. 
Upon  coming  in  contact  with  any  substance,  light  undergoes 
one  or  more  of  the  following  modifications: 

(1)  Reflection:  it  is  turned  back  by  the  surface  of  the 
object  or  medium. 

(2)  Refraction  :  it  passes  into  and  through  the  substance  or 
medium,  undergoing  changes  of  velocity  and  direction. 

(3)  Absorption :  its  waves  are  wholly  or  partially  ex- 
hausted in  passing  through  the  substance,  so  that  it  does  not 
emerge  on  the  other  side. 

(4)  Dispersion  :  it  is  broken  up  into  its  composite  parts  by 
refraction  by  a  medium  whose  incident  and  emergent  surfaces 
are  not  parallel  to  each  other. 

(5)  DifYraction :  it  is  split  into  its  component  parts  by 
impingement  on  a  sharp  edge  of  a  hard  substance. 

(6)  Polarization  :  By  double  refraction  through  a  bi-axial 
crystal. 

Each  of  these  phenomena  of  light  is  fully  dealt  with  under 
its  own  heading,  to  which  the  reader  is  referred. 

WAVE  CURVATURE. 

The  ophthalmic  refractionist  deals  almost  wholly  with  the 
light  waves  of  Huygens  in  isotropic  media,  whose  curvature 
is  spherical ;  and  this  quality  of  curvature,  together  with  that 
of  velocity,  constitute  the  very  foundation  of  eye  refraction. 
The  curvature  of  a  wave  varies  inversely  as  its  radius,  and  these 
two  quantities  are  therefore  reciprocals  of  each  other.  The  unit 
of  curve  measurement,  as  used  by  the  refractionist.  is  the 
curvature  of  a  wave  ha\ing  a  radius  of  1  meter;  this  is  known 
as  the  meter  curve,  or  mc.  All  other  curvatures  are  expressed 
in  multiples  or  dividends  of  the  meter  curve  or  its  radius.  If  r 
stands  ior  the  radius,  then  : 

1 
mc  =  — 

r 

SOURCES  OF  LIGHT. 
Light  that  reaches  us  from   the  sun  is  called   natural  li^iit  ; 
that  which  is  furnished  by  artificial  means  is  known  as  artifi- 
cial light.     There  are  two  principal  modes  of  producing  arti- 
ficial light,  viz.,  cond)ustion,  as  exemplilied  in  the  candle,  oil 


1 


LIGHT  249 

lamp,  and  gas  flame,  and  friction,  as  seen  in  electric  light.  This 
aspect  of  the  subject  will  be  found  discussed  in  the  section  on 
Illumination. 

SUMMARY. 
The  theory  of  light  may  be  summed  up  in   the  following 
propositions   formulated   by  Thomas  Young  in   his   Bakerian 
lecture  of  1801 : 

(1)  A  luminiferous  ether  pervades  the  universe,  rare  and 
elastic  in  a  high  degree. 

(2)  Undulations  are  excited  in  this  ether  whenever  a  body 
becomes  luminous. 

(3)  The  sensation  of  different  colors  depends  upon  the  dif- 
ferent frequency  of  vibrations  excited  by  the  light  on  the 
retina. 

(4)  All  material  bodies  have  an  attraction  for  the  etherial 
medium,  by  means  of  which  it  is  accumulated  in  their  sub- 
stances and  for  a  small  distance  around  them  in  a  state  of 
greater  density  but  not  of  greater  elasticity. 

(5)  All  impulses  are  propagated  in  a  homogeneous  elastic 
medium  with  an  equable  velocity. 

(6)  An  undulation  conceived  to  originate  from  a  single 
particle  must  expand  through  a  homogeneous  medium  in  a 
spherical  form,  but  with  different  quantities  of  motion  in  dif- 
ferent parts. 

(7)  A  portion  of  a  spherical  undulation,  admitted  through 
an  aperture  into  a  quiescent  medium,  will  proceed  to  be  further 
propagated  rectilinearly  in  concentric  superfices,  terminated 
laterally  by  weak  and  irregular  potions  of  newly  diverging  un- 
dulations. 

(8)  When  an  undulation  arrives  at  a  surface  which  is  the 
limit  of  media  of  different  densities,  a  partial  reflection  takes 
place,  proportionate  in  force  to  the  difference  of  densities. 

(9)  When  an  undulation  is  transmitted  through  a  surface 
terminating  dift'erent  media,  it  proceeds  in  such  a  direction 
that  the  sines  of  the  angles  of  incidence  and  refraction  are  in 
the  constant  ratio  of  the  velocities  of  propagation  in  the  two 
media. 


250  LIGHT  AREA 

(10)  When  an  undulation  falls  on  the  surface  of  a  rarer 
medium  so  obliciuely  that  it  cannot  he  regularly  refracted,  it 
is  totally  reflected  at  an  angle  equal  to  that  of  its  incidence. 

(11)  If  equidistant  undulations  be  supposed  to  pass  through 
a  medium  of  which  the  jjarts  are  susceptible  of  permanent  vi- 
brations somewhat  slower  than  the  undulations,  their  velocity 
will  be  somewhat  lessened  by  this  vibratory  tendency,  and, 
in  the  same  medium,  the  more  as  the  undulations  are  more 
frequent. 

(12)  When  two  undulations  from  different  origins  coin- 
cide, either  perfectly  or  very  nearly,  in  direction,  their  joint 
effect  is  a  combination  of  the  motions  belonging  to  each. 

(13)  Radiant  light  consists  in  undulations  of  the  lumin- 
iferous  ether. 

EINSTEIN'S  DOCTRINE. 
Within  the  last  year  (1920)  Albert  I'^instein,  of  Germany, 
has  propounded  the  doctrine  that  light  is  possessed  of  mass, 
and  is  subject  to  gravitation,  i.  e.,  it  is  attracted  by  large  bodies 
and  deflected  out  of  its  straight  path.  He  not  only  stipulated 
its  deflection,  but  calculated  the  degree  to  which  it  would  be 
deflected  by  a  gi\en  mass  and  density  of  body  ;  and  this  was 
verified  both  qualitatively  and  (|uantitatively  l)y  a  commission 
of  the  Royal  Society  of  Great  Britain  during  the  last  solar 
eclipse. 

Light  Area.    Term  applied  to  the  light  thrown  by  the  retinoscope 
on  the  face  and  in  the  pupil. 

Limbus.      Tb-c  border-line  between  tiie  cornea  and  the  sclera. 

Limbus  Cornea.      The  junction  of  cornea  ami  sclera. 

Limit  Angle.     Sre  Critical  Angle. 

Line  of  Fixation,    .^ee  Fixation. 

Line  of  Vision.     A  collocpiial  term  for  the  visual  axis. 

Lippitudo.     Same  as  Blepparitis  Marginalis. 

Lippus,     I'k-ar  eyed. 

Liquor  Morgagni.     .\  thin  layer  of  fluitl  brtween  the  crystalline 
lens  and  its  capsule. 


LITHIASIS  251 

Lithiasis.  Calcarous  deposits  in  secretion  of  the  Meibomian 
glands. 

Logadectomy,  •  Removal  of  a  portion  of  the  conjunctiva  with  a 
knife. 

Logades.    The  first  tunic  of  the  eye. 

Loimophthalmia.     Contagious  ophthalmia. 

Long-Sightedness.  A  common  name  for  hyperopia,  referring  to 
the  ability  of  the  hyperope  to  see  better  at  a  distance  than  at 
near  point. 

Lorgnette.     Folding  eye-glasses  attached  to  a  handle. 

Lorgnon.     Same  as  lorgnette. 


m 


Lorgnon. 

Louchettes.  Opaque  glasses  with  a  small  opening  for  each  eye 
which  forces  the  patient  to  look  through  this  opening. 

Loupe.     A  rnagnif3'ing  lens  used  for  examining  the  eye. 
Loxophthalmos.     Strabismus. 

Lucifugal.    A  condition  in  which  the  patient  avoids  Ijright  light. 

Luminous  Body.  A  luminous  body  is  one  which  is  a  source  of 
light.  They  may  be  divided  into  natural  and  artificial.  The 
sun  is,  of  course,  the  only  natural  source  of  light  known  to  us ; 
candles,  lamps,  electric  lights,  are  artificial  luminous  bodies. 

The  modern  viewpoint,  however,  regards  many  reflecting 
bodies  as  luminous  bodies,  holding  that  they  so  change  the 
quality  of  the  light  waves  that  strike  them  as  to  become  prac- 
tically sources  of  new  light  waves.  Thus,  we  can  certainly 
regard  the  moon  as  a  true  luminous  body,  although  its  light 
is  reflected  from  the  sun.  In  the  same  way,  we  regard  the 
retina  as  a  true  source  of  light  in  our  retinoscopic  and  ophthal- 
moscopic work. 


252  LUXATIO   BULBI 

Luxatio  Bulbi.    Avulsion  of  the  eyeball. 

I-uxation.  Another  word  for  dislocation.  In  ophthalmology  it 
refers  to  a  dislocation  of  the  crystalline  lens. 

Lymph.  A  fluid  in  the  body  which  forms  an  intermediary  be- 
tween the  blood  and  the  tissues  which  the  blood  is  to  nourish, 
and  may  be  regarded  as  a  sort  of  reservoir  for  the  nutriment 
which  the  blood  brings  to  these  tissues  and  also  for  the  waste 
substances  which  the  tissues  give  up  for  the  blood-stream  to 
remove.  Through  this  fluid,  therefore,  the  exchange  between 
the  blood  and  the  tissues  is  constantly  taking  place.  It  is  a 
colorless,  transparent  fluid,  contained  in  a  system  of  vessels 
and  glands,  very  similar  to  the  system  of  blood-vessels,  and  is 
eventually  poured  into  the  venous  system  by  means  of  a  large 
lymph-\'essel  called  the  thoracic  duct,  near  the  heart. 

Lymphadenoma.     A  tumor  of  the  lymphoid  tissue. 

Lymphatics.  A  general  name  given  to  the  system  of  vessels  and 
glands  concerned  in  the  elaboration  and  transportation  of 
lymph.  The  lymphatic  glands  are  very  important  structures, 
for  it  is  only  after  passing  through  them  that  the  lymph  is 
ready  and  fit  to  enter  the  blood.  Their  average  size  is  that  of 
an  almond,  and  they  are  usually  arranged  in  groups;  but  many 
of  them  are  very  much  smaller.  The  lymphatic  vessels  which 
absorb  nutriment  from  the  intestines  are  called  the  lacteals. 
The  eye,  oi  course,  has  its  lymphatic  system. 

Macrocornea.  Megalocornea.  Cornea  of  abnormal  size  or 
projection. 

Macrophthalmus.    Indicating  unusually  large  eyes. 

Macropsia.  A  condition  in  which  objects  appear  larger  than 
normal. 

Macroscopic.     Large  enough  to  be  seen  with  the  naked  eye. 

Macula  Lutea.    The  yellow  spot.     The  mcjst  sensitive  spot  of  the 

retina,  situated  about  a  disc  diameter  from  the  disc,  toward  the 

temporal  side.    It  appears  yellow  because  it  is  de\ oid  of  vessels, 

so  that  the  pigment  of  the  chorioid  sIkjws  uninterruptedly. 

0])tically,  the  yellow  spot  is  the  center  of  the  visual  field 


\ 


MADAROSIS  253 

on  the  retina.  Only  that  portion  of  an  object  which  is  repre- 
sented by  foci  on  the  yellow  spot  is  viewed  with  attention.  It 
is  the  yellow  spot  which  is  directed  toward  an  object  in  fixa- 
tion, and  a  line  from  the  yellow  spot  to  the  object  constitutes 
the  visual  axis. 

In  the  center  of  the  macula  is  the  fovea  centralis,  the  most 
sensitive  point  in  the  macula. 

Madarosis.    Complete  destruction  of  the  eye-lashes. 

Maddox  Rod.  An  apparatus  devised  by  Dr.  Ernest  Maddox,  of 
England,  for  testing  muscular  imbalance.  It  consists  of  an 
opaque  disc  with  a  glass  rod  set  in  it,  which  draws  out  the 


Maddox  Rod. 

image  of  a  candle-flame  or  circular  light  into  a  streak  at  right 
angles  to  the  rod,  so  as  to  dissociate  it  from  the  image  seen 
by  the  other  eye.    For  description  of  its  use  see  Heterophoria. 

Madisterium.    An  instrument  for  removing  eye-lashes. 

Magnification.  Magnifying  Power.  The  ratio  between  the  angle 
subtended  by  the  object  at  the  center  of  the  entrance-pupil  of 
a  lens  system  and  the  linear  dimension  of  the  image,  in  favor 
of  the  image. 

Intrinsic  Magnification.  The  ratio  between  the  visual  angle 
subtended  at  the  nodal  point  of  the  eye  by  the  image  viewed 
through  the  magnifying  instrument  and  the  corresponding 
linear  dimension  of  the  object. 

Objective  Magnification.  The  ratio  of  the  linear  size  of  the 
image  to  the  apparent  size  of  the  object. 

Subjective  Magnification.    The  ratio  of  the  visual  angles  or 


254  MAGNIFY 

their  tangents  subtended  at  the  nodal  point  of  the  eye;  this 
ratio  involves  the  distance  of  distinct  vision. 

Magnify.     To  increase  the  apparent  size  of  an  object. 

Malacocataract.  A  soft  cataract,  usually  traumatic,  occurring  in 
persons  under  forty. 

Malaxation.     Massage  of  the  eye. 

Malignant.     Tending  to  fatality. 

Malingering.  Pretending  to  some  form  of  sickness,  or  physical 
or  mental  defectiveness,  usually  for  some  interested  purpose, 
such  as  avoiding  military  service,  recovering  damages  for  an 
alleged  injury,  etc.  In  ophthalmology  malingering  generally 
implies  the  pretense  to  some  degree  of  blindness  or  defective 
vision.  There  are  many  tests  for  the  detection  of  such  malin- 
gering, of  which  the  following  are  the  most  commonly  em- 
ployed : 

Bar  Test.  A  pencil  or  other  similar  object  is  interposed 
between  the  patient's  eyes  and  reading  type  at  a  suitable  dis- 
tance. If  he  is  reading  with  one  eye  only,  this  will  shut  off  a 
part  of  the  type ;  but  if  he  is  using  both  eyes,  he  can  read 
around  it. 

Convex  Lens  Test.  A  convex  lens  of  6  D.  is  placed  before 
the  sound  eye.  This  places  the  far  point  at  about  16  cm.  A 
reading  chart  of  suitable  type  is  held  well  within' this  far  point, 
and  the  patient  reciuired  to  read  aloud.  As  he  reads,  the  chart 
is  gradually  withdrawn  beyond  the  16  cm.  If  he  still  continues 
to  read  the  letters,  it  is  exidcnt  he  is  doing  it  with  thi-  other 
eye. 

Prism  Test.  While  the  patient  is  reading,  with  both  eyes 
open,  we  make  a  show  of  concerning  ourselves  with  the  sound 
eye.  As  he  reads,  we  slowly  interpose  a  prism,  about  4 
diojjtres,  base  up,  before  the  sound  eye,  gradually  pushing  it 
upward  in  front  of  the  i  yr,  until  its  b.isi-  cuts  the  center  of 
the  pupil.  This  ])rotluces  monocular  diplopia,  to  which  the 
])atient  will  readily  ass'.nl.  Continuing  the  test,  we  push  the 
prism  still  further  up  until  it  covers  the  sound  eye.  Then- 
will  now  1)1'  no  more  mon<iciiI;ii'  diplopi.i.  but  binocular  diplo- 


MANN'S  SIGN  255 

pia.  If,  therefore,  the  patient  still  admits  double  vision,  he  is 
reading  with  both  eyes. 

Complementary  Color  Test.  Snellen's  test  type,  with  let- 
ters alternately  red  and  green,  is  used.  A  green  glass  is  placed 
before  one  eye  and  a  red  glass  before  the  other,  in  the  trial 
frame.  The  green  glass  blots  out  the  red  letters,  being  com- 
plementary colors,  and  the  red  glass  the  green  letters.  If, 
therefore,  the  patient  reads  all  the  letters,  it  is  evident  that 
both  eyes  are  in  vision. 

Black  and  Red  Test.  A  variation  of  the  above  test  is  to 
use  alternately  black  and  red  letters  on  a  white  background.  A 
red  glass  is  placed  before  the  sound  eye,  which  makes  the 
background  substantially  the  same  color  as  the  red  letters. 
The  patient  is  made  to  read  the  letters  rapidly,  ancl  if  he  reads 
them  all,  it  is  evident  that  he  is  doing  so  with  the  alleged  poor 
eye. 

In  any  of  these  tests  the  patient,  if  he  be  shrewd,  may  fool 
the  examiner  by  slyly  closing  the  alleged  poor  eye  for  a  moment 
and  getting  an  idea  how  the  chart  ought  to  be  read.  We  can 
meet  this  by  suddenly  placing  a  4  dioptre  prism,  base  up,  be- 
fore alleged  poor  eye,  while  he  is  reading  aloud  with  both  eyes 
open.  If  the  eye  be  really  poor,  the  prism  will  have  no  efifect 
and  he  will  read  right  on  ;  if  it  be  a  good  eye,  the  sudden 
doubling  of  the  images  will  confuse  him  in  his  reading. 

Mann's  Sign.     Condition  in  exophthalmic  goiter  when  two  eyes 
do  not  seem  to  be  on  a  level. 

Marginal.     Occurring  at  the  margin  of  an  area. 

Marginal  Blepharitis.  Inflammation  of  the  margin  of  the 
eyelid. 

Marginal  Keratitis.  Inflammation  around  the  margin  of  the 
cornea,  occurring  in  old  persons. 

Mariotte's  Blind  Spot.    The  optic  disc. 

Mariotte's  Experiment  is  the  demonstration  of  the  blind 
spot  by  means  of  a  dot  and  a  cross,  several  inches  apart,  on  a 
card ;  as  the  card  is  moved  to  or  from  the  eye,  with  the  eye 
fixed  on  the  dot,  the  cross  becomes  invisible  at  a  certain  dis- 
tance, proving  the  existence  of  the  blind  spot. 


256  MARMARYGEA 

Marmarygea.     Appearance  of  sparks  before  the  eyes. 

Mature.     Fully  ripe ;  applied  to  a  cataract. 

Mechanics  of  Fitting  Eyes.     See  Frame  Fitting. 

Median.     A  technical  term  for  middle. 

Median  Line.  In  physiologic  optics  usually  applied  to  the  line 
drawn  perpendicularly  to  that  which  joins  the  geometric  cen- 
ters of  the  two  eyes. 

Medium.  In  optics  this  word  specifically  designates  a  body  or 
substance  through  wiiich  light  passes  and  by  which  it  is  in- 
fluenced in  passing — principally  refracting  bodies  or  sub- 
stances,— as  distinct  from  their  surfaces. 

The  media  of  the  eye  are  the  cornea,  the  aqueous  humor, 
the  crystalline  lens  and  the  vitreous. 

Megalocornea.    Bulging  of  the  cornea. 

Megalophthalmos.     An  unusually  large  size  of  eye. 

Megalopsia.  A  condition  in  which  objects  appear  larger  than 
they  really  are. 

Meibomian  Glands.  Acinous  glands  embedded  in  the  tarsal 
bodies  of  the  eyelids.  They  secrete  an  oily  (sebaceous)  fluid 
which  serves  to  lubricate  the  lids  and  the  eyeball. 

Melanin.     A  dark  pigment  of  the  choroid. 

Melanocataracta.     l>lack  cataract. 

Melanophthalmous.  Melanoma  of  the  eye.  Black  eyes.  In 
botany  indicates  s])ots  surrcnmded  by  black  circles,  resembling 
eyes. 

Melasma  Palpebrarum.     I  )isc()li)ratiun  of  the  eyelid. 
Meliceris.     C'hala/ion. 

Membrane.      .\   thin   coxerinj^    tissue. 

Capsular  Membrani".  .\  vascular  network  o\er  the  pt^sterior 
surface  of  the  lens. 

Nictating  Membrane.  .\n  extra  eyelid,  seen  in  birds  and 
other  lower  animals. 


MENISCUS  LENS  257 

Pupillary  Membrane.  A  membrane  covering  the  pupil  in 
fetal  life,  which  occasionally  persists. 

Meniscus  Lens.     See  Lens. 

Menotyphlosis.     Diminution  of  vision  during  the  night. 

Meramaurosis.    Partial  amaurosis. 

Meridian.  The  word  is  derived  from  a  Latin  word  meaning 
pertaining  to  the  south,  or  midday,  and  was  originally  applied 
to  an  imaginary  line  drawn  around  the  earth  from  the  north 
pole  through  the  south  pole.  It  was  later  extended  to  mean 
any  imaginary  line  drawn  around  a  sphere,  so  as  to  divide  it 
equally,  in  any  of  the  360  angular  directions.  Also  to  diam- 
eters of  a  circle  at  any  of  these  angles. 

As  we  usually  represent  a  sphere,  diagrammatically,  by  a 
plane  circle,  so  also  we  represent  its  meridians  as  meridians 
of  a  circle.  They  are  numbered  from  the  left-hand  extremity  of 
the  horizontal  equator  in  the  direction  of  the  figures  on  a  clock- 
dial,  the  extreme  left  of  the  equator  being  0,  the  vertical  90 
degrees,  the  extreme  right  of  the  equator  180  degrees,  the 
lower  vertical  270  degrees,  etc. 

As  to  the  significance  of  the  meridians  of  the  cornea  in 
astigmatism,  see  Astigmatism. 

It  should  be  borne  in  mind,  in  subjective  testing,  that  the 
meridians  on  either  side  the  vertical  on  the  test-chart  repre- 
sent meridians  on  the  opposite  side  of  the  cornea. 

Meropia.     See  Amblyopia. 

Mesiris.     Substantia  propria  of  the  iris.     The  middle  layer. 

Mesoretina.     The  middle  layer  of  the  retina. 

Mesoropter.    The  position  of  the  eyes  in  a  state  of  rest. 

Mesoseme.     Having  a  medium  orbital  index  of  85  to  90. 

Metamorphopsia.  A  condition  in  which  objects  appear  dis- 
torted. 

Meter  Angle.  The  angle  made  by  the  visual  axes  with  the  middle 
perpendicular  line.    See  Convergence, 


258  METER  CURVE 

Meter  Curve.  The  curvature  of  a  sphere,  or  segment  thereof. 
ha\  ing  a  radius  of  1  meter.  The  unit  of  curvature  in  optical 
mathematics. 

Metric  System.  Also  called  Decimal  System.  A  system  of 
weight,  measure,  and  money,  in  which  the  unit  is  multipled  \>y 
10  or  some  power  of  10  to  give  the  higher  denomination,  and 
divided  by  10  or  a  power  of  10  to  give  the  lower  denominations. 
So  far  as  optics  is  concerned,  the  metric  system  is  a  decimal 
method  of  lineal  measurement,  of  which  the  meter  is  the  unit 
(being  equal  to  39.371  inches).  Latin  prefixes  are  used  to 
denote  dividends  of  the  meter,  and  (ireek  prefixes  to  indicate 
multiples,  as  follows: 

Decimeter.  ..  .A  tenth  of  a  meter. 

Centimeter..  .  .A  hundredth  of  a  meter. 

Millimeter.  ..  .A  thousandth  of  a  meter. 

Decameter..  .  .Ten  meters. 

Hektometer..  .One  hundred  meters. 

Kilometer One  thousand  meters. 

The  metric  system  of  weights  and  measures  is  used  almost 
entirely  by  scientists,  because  of  its  simplicity,  and  the  ease 
with  which  it  adapts  itself  to  unit  calculations.  In  optics  this 
is  readily  seen  in  the  reciprocity  between  the  radius  and  the 
curvature  of  a  light  wave,  and  between  tlie  f(xal  length  and 
dioptric  power  of  a  mirror  or  lens. 

Meyer's  Rings.  Rings  of  \iolet  or  blue  seen  around  a  candle 
tlanie  against  a  dark   background,  due  tcj  ditYracti«in. 

Microblepharism.     Smallness  of  the  e\elids. 

Microcornea.     An  c-xceptiijnally  small  si/e  of  the  cornea. 

Microlentia.    An  unusually  small  si/c  of  the  cryslailinc  lens. 

Micrometer.  An  instrument  for  measuring  very  small  arcs  in 
the  field  of  the  telescoi)f,  antl  for  making  very  small  linear 
measurements  in  other  departments  of  physics. 

Micron.     A  milliontli  |);irt  i»f  a  niiliimi-ter. 

Microphakia.      An    unusually    small   si/e  of   the  crystalline   lens. 


MICROPHTHALMIA  259 

Microphthalmia.    Abnormal  smallness  of  the  eyeballs. 

Micropsia.  A  condition  in  which  everything  appears  smaller 
than  normal. 

Microscope.  An  instrument  for  magnifying  the  image  of  minute 
objects.  It  consists  of  a  combination  of  lenses,  and  in  some 
cases  of  mirrors  also,  one  lens,  or  combination  of  lenses  form- 
ing the  objective,  to  magnify  the  image,  and  one  lens  forming 
the  eyepiece,  for  focussing  the  magnified  image  on  the  ob- 
server's retina.  The  principle  of  magnification  is  that  the  rela- 
tive sizes  of  object  and  image  are  directly  as  their  relative 
distance  from  the  optical  center  of  the  objective  lens.  Thus 
the  magnifying  power  of  a  microscope  can  be  increased  by  (1) 
increasing  the  strength  of  the  objective  lens,  (2)  increasing  the 
power  of  the  eyepiece  lens,  or  (3)  increasing  the  distance  be- 
tween objective  and  eyepiece. 

Milium.     A  seed-like  elevation  under  the  skin  of  the  eyelid. 

Milphae.     Dropping  out  of  the  eyelashes. 

Milphosis.     Falling  out  of  the  lashes  and  eyebrows. 

Minus.  A  negative  quantity,  i.  e.,  less  than  nothing.  As  ap- 
plied to  optics,  it  denotes  a  wave-curve  or  a  lens-power  whose 
surface  is  concave. 

Mirror.  A  polished  surface  which  reflects  practically  all  the 
light  that  strikes  it.  Such  a  reflecting  surface  may,  of  course, 
have  any  kind  of  configuration,  regular  or  irregular.  In  op- 
tics we  have  to  do  with  plane  and  curved  mirrors ;  and  in  ele- 
mentary optics  the  curved  mirrors  are  spherical,  either  convex 
or  concave. 

Plane  mirrors  merely  change  the  direction  of  propagation 
and  turn  the  front  of  the  light  waves  that  strike  them ;  they 
exercise  no  influence  upon  the  curvature  of  the  waves. 

Spherically  curved  mirrors  add  twice  their  own  curvature 
to  the  curvature  of  the  light  wave  that  strikes  them,  as  well 
as  turning  their  front  and  changing  the  direction  of  their  travel. 
Thus,  a  concave  mirror,  adding  twice  its  own  curvature  (which 
is  minus)  to  a  neutral  wave,  turns  it  into  a  minus  wave  of 


260  MIRROR 

twice  its  own  curvature,  and  brings  it  to  a  focus  in  half  the 
radius  of  the  mirror.  Tlie  focal  length  of  a  mirror,  therefore, 
is  half  its  radius;  and  its  dioptrism  is  twice  its  physical  or 
meter  curve. 

If  the  wave  that  strikes  a  conca\  e  mirror  comes  from  a  point 
within  infinity  (a  plus  wa\e).  the  mirror  adds  its  dioptrism  to 
the  wave,  which  turns  it  into  a  minus  wave  equal  to  the 
dioptrism  of  the  mirror,  less  its  own  plus  curvature  when  it 
struck,  and  brings  it  to  a  focus  at  a  distance  corresponding  to 
the  radius  of  curvature  of  the  wave  when  it  leaves  the  mirror. 
Thus,  if  the  wa\  e  originates  50  cm.  from  a  2.50  D.  concave 
mirror,  it  will  be  a  plus  2D.  wave  when  it  strikes  the  mirror; 
the  mirror  will  add  minus  2..^0  D.  to  it.  making  it  a  minus  .50  D. 
when  it  leaxes,  and  it  will  focus  at  2  meters  ( which  is  the  radius 
of  a  .50  D  wave)  in  front  of  the  mirror. 

If  the  \\a\e  originates  at  the  spherical  center  of  the  mirror, 
the  reflected  wave  will  be  so  curved  as  to  trace  exactly  the 
same  path  as  the  incident  wave,  and  the  object  and  the  image 
will  occupy  the  same  point  in  space. 

If  the  incident  wave  originate  nearer  to  tlic  mirror  than  its 
own  focal  length,  the  mirror  can  no  longer  turn  it  into  a  minus 
wave,  but  only  reduce  its  convexity. 

The  image  formed  by  the  reflection  from  a  concave  mirrcjr 
is,  therefore,  a  real,  inverted  image  of  the  object  if  the  latter 
lie  outside  the  optical  center  of  the  mirror;  identical  with  the 
object  if  the  ol)ject  lie  at  the  center;  and  a  virtual,  upright 
image  if  the  object  lie  inside  the  focal  length  of  the  mirror. 

A  convex  mirror,  adding  its  own  dioptrism  to  a  li^hl  \\a\c. 
cannot  bring  either  neutral  or  convex  wa\es  to  a  fttcu>.  but 
renders  them  still  more  plus  by  just  its  own  dioptrism.  l'\)r 
the  purposes  of  measurcnuut,  however,  siuh  waves,  as  they 
lea\e  the  mirror,  are  ])rojected  backwaril  to  the  pc»int  from 
which  the\  appear  to  ha\e  originated,  and  tiiis  is  said  to  be 
thrir  negative  focus.  C Dnv  ex  mirrors  thus  lia\  e  negati\  e  focal 
points,  an<l  virtual  images.  They  do  wui  exist  except  in  tlie 
mathematician's  mind.  Their  mathematical  relation>,  how - 
e\er,  arc  the  same  as  in  tlu-  case  ot  concave  mirrors,  except 
that  the  negative  point>  all  lie  biliind  the  mirror  instead  of  in 
front  of  it. 


MIRROR  261 

Real  images  are  conjugate  with  the  objects  they  reproduce, 
because  if  the  situation  were  reversed,  and  the  object  were 
placed  where  the  image  is,  the  image  would  be  made  in  the 
place  where  the  object  is.  Virtual  images,  of  course,  are  not 
conjugate  with  any  point,  because  they  do  not  really  exist.  If 
the  object  were  placed  where  a  virtual  image  is  supposed  to 
be,  an  entirely  different  set  of  optical  c6nditions  would  be 
created. 

From  what  has  been  said,  it  will  be  seen  that  the  dioptric 
power  of  a  spherical  mirror  is  twice  its  metric  curve,  which  it 
adds  to  the  light  wave  that  strikes  it. 

When  light  falls  on  a  spherical  mirror  beyond  a  certain 
acuteness  of  angle  it  is  not  reflected  in  accordance  with  the 
laws  of  reflection. 

(See  Reflection.) 

DOUBLE  MIRRORS. 

If  two  plane  mirrors  be  opposed  to  each  other,  with  their 
surfaces  parallel,  and  a  luminous  point  between  them,  the  num- 
ber of  images  formed  is  infinite,  all  lying  upon  a  line  passing 
through  the  luminous  point  and  perpendicular  to  the  mirrors. 

If  two  plane  mirrors,  with  their  surfaces  opposed,  be  in- 
clined at  an  angle,  then  it  is  evident  that  the  line  on  which  the 
images  lie  is  not  a  straight  line,  but  the  arc  of  a  circle,  whose 
center  is  the  point  of  intersection  of  the  two  mirrors  and  whose 
circumference  passes  through  the  luminous  point.  The  num- 
ber of  images  is  now  manifestly  limited,  for  when  any  image 
falls  in  the  arc  lying  between  the  planes  of  the  two  mirrors 
produced  beyond  the  point  of  intersection,  this  image  lies  be- 
hind both  mirrors,  and  hence  no  further  image  is  possible. 
The  arc  in  question  is,  of  course,  equal  in  angular  measurement 
to  the  arc  between  the  two  mirrors,  and  the  portion  of  it  which 
is  "dead"  for  each  mirror,  respectively,  is  equal  to  the  portion 
lying  between  the  mirror  in  question  and  the  luminous  point. 
If  6  stands  for  the  angle  of  separation,  d  for  the  distance  be- 
tween object-point  and  mirror,  and  tt  for  the  value  of  two  right 
angles,  then  the  number  of  possible  images  for  each  mirror  is 
represented  bv  the  integral  number  next  greater  than 
(;r-d)/^. 


262  MIOSIS 

If  ir/B  be  itself  an  integral  number,  then  the  number  of 
images  for  each  series  is  tt  B,  but  an  image  of  each  series  co- 
incides;  hence  the  number  of  images,  including  the  object- 
point,  is  2it/B. 

Thus,  if  two  mirrors  be  inclined  at  an  angle  of  70  deg.,  and 
the  object-point  be  20  deg.  from  one  surface  and  50  deg.  from 
the  other,  the  number  of  images  in  the  first  series  will  be  equal 
to  180 — 20/70  +  sufficient  to  make  the  next  greater  integer, 
i.  e.,  3;  in  the  second  series,  180 — 50/70  plus  enough  to  make 
the  next  integer,  i.  e.,  2;  total  number  of  images,  5. 

If  the  angle  of  inclination  be  60  deg.,  then  tt/^  is  itself  an 
integer,  and  the  number  of  images  will  be  360/60  =  6,  includ- 
ing the  object-point,  or  5  true  images.  Advantage  is  taken  of 
this  latter  principle  in  the  construction  of  the  kaleidescope,  in 
which  7  images  are  produced  by  an  inclination  of  45  deg. 

Miosis.  Forced  contraction  of  the  pupil  by  means  of  a  drug. 
The  two  drugs  commonly  used  for  this  purpose  are  eserine  (or 
physostigmine)  and  pilocarpine.  Contraction  of  the  pupil  is 
brought  about  by  contraction  of  the  sphincter  iridis  muscle; 
and  both  these  drugs  put  this  muscle  into  a  state  of  tonic  con- 
traction. It  should  be  borne  in  mind  that  they  also  affect  the 
ciliary  muscle  in  the  same  way,  producing  an  artificial  accom- 
modation. 

Miotic.    A  drug  that  produces  miosis.    See  above. 

Moebius'  Disease.  Periodic  paralysis  of  the  muscles  of  the  ovc, 
with  migraine. 

Monoblepsia.  A  condition  in  which  the  sight  i>f  one  eye  is  much 
better  than  that  of  both  together. 

Monocentric.  Originating  from  one  center.  Same  as  Homo- 
centric. 

Monochromasia.     Color-blindness  to  all  but  one  color. 

Monochromatic.  .Applied  to  a  beam  of  lif^ht  in  which  tlicrc  is 
but  one  length  of  wave. 


MONOCHROMATIC  ABERRATION  263 

Monochromatic  Aberration.  Spherical  aberration.  See  Aberra- 
tion. 

Monocle.   A  single  eyeglass. 

Monocular.  The  terms  refers  to  any  function  or  appliance  in 
which  one  eye  alone  participates. 

Monocular  accommodation.  The  accommodation  exerted 
when  only  one  eye  is  in  vision. 

Monocular  diplopia.  Double  image  seen  with  one  eye  alone. 
Usually  due  to  astigmatic  facets  on  the  cornea. 

Monocular  squint.     Deviation  of  one  eye  only. 

Monocular  vision.    Vision  with  one  eye  alone. 

Monops.    A  monster  with  only  one  central  eye. 

Moon-Blindness.  A  rare  form  of  retinal  blindness  due  to  sleep- 
ing in  the  bright  tropical  moonlight. 

Morgagnian  Cataract.  A  cataract  which  has  shrunk  so  that  there 
is  a  layer  of  fluid  between  the  lens  and  its  capsule. 

Morgagnian  Humor.  A  thin  layer  of  fluid  supposed  to  lie  be- 
tween the  crystalline  lens  and  its  capsule. 

Mucocele.    Any  distension  of  a  sac  as  of  the  lacrymal  sac. 

Mueller's  Muscle.    The  sphinctre  muscle  of  the  eye. 

Murine.     Proprietary  eyewater  popular  with  the  public. 

Muscae  Volitantes.  Floating  spots  seen  subjectively.  They  are 
the  projection  of  minute  particles  in  the  vitreous  humor.  They 
may  be  normal  or  pathologic.  Myopes  frequently  see  them, 
because  of  the  abnormal  length  of  the  eye-ball.    See  Entoptic. 

Muscle  Exercises.  The  exercising  of  muscles,  in  general,  can  be 
divided  into  two  classes,  (1)  sheer  physical  exercising  of  the 
muscle  itself,  for  the  purpose  of  increasing  its  bulk  and  power, 
and  (2)  exercising  of  the  co-ordinating  faculty  of  physiologic 
groups  of  muscles. 

In  prescribing  and  carrying  out  exercises  of  the  extrinsic 
muscles  of  the  eyes,  for  the  cure  of  imbalance  or  squint, — es- 


264  MUSCULAR  ASTHENOPIA 

pecially  in  cases  of  imbalance — it  is  doubtful  if  the  former  type 
of  exercise  is  ever  of  any  value.  It  is  highly  probable  that 
whatever  exercises  are  employed  derive  their  value  (if  they 
prove  useful)  from  the  training  which  they  give  the  patient 
in  the  co-ordinated  use  of  his  muscles  in  pairs,  in  subservience 
to  the  fusion  faculty.  In  the  performance  of  such  exercises, 
of  course,  there  is  an  incidental  strengthening  of  all  the  muscles 
of  the  eye. 

The  internal  recti  (which  are  most  amenable  to  treatment) 
may  be  exercised  either  with  or  without  the  aid  of  prisms. 
Simply  by  following  with  the  eyes  an  object  gradually  brought 
in  from  infinity  to  near-point,  or.  (what  is  the  same  thing), 
slowly  walking  toward  an  object  with  the  eyes  fixed  upon  it. 
the  power  of  convergence  may  be  trained  and  strengthened. 
Or  by  placing  before  the  eyes  successively  and  gradually  in- 
creasing prisms,  base  out,  while  the  patient  fixes  an  object  at 
infinity,  the  same  effect  can  be  obtained.  The  prism,  or  prisms, 
can  be  increased  until  diplopia  is  produced.  Such  exercise, 
however,  should  never  be  carried  beyond  the  point  of  fatigue. 

What  are  known  as  rotary  prisms  are  nowadays  available 
for  this  purpose,  consisting  of  a  pair  of  prisms  rotated  on  each 
other  by  means  of  a  set-screw  handle,  so  that  the  prism  power 
of  the  combination  can  be  increased  or  reduced  in  a  continuous 
sliding  scale.  This  has  a  much  better  exercising  effect  than  the 
sudden  jumping  from  one  prism  power  to  another. 

The  external  recti  can  be  exercised  only  with  the  aitl  of 
prisms,  because  there  is  no  way  of  contracting  tlu-in  noIuii- 
tarily.  Prisms,  base  in,  will  force  the  use  of  the  externals. 
However,  as  this  is  a  forced  use  of  the  muscles,  such  as  does 
not  take  place  in  ordinary  physiologic  function,  it  is  highly  ini- 
j>rol)able  that  it  does  any  good. 

After  all  is  said  and  done,  the  indirect  use  of  the  muscles, 
both  internal  and  external,  which  is  obtained  in  the  education 
of  the  fusion  faculty,  with  stereoscopes,  amblyscopes.  etc..  is 
the  best  that  can  be  gi\en. 

Muscular  Asthenopia.     See  Asthenopia. 

Muscular  Imbalance.     Sec  Heterophoria. 


MYCOPHTHALMIA  265 

Mycophthalmia.     Spongy  growth  of  the  conjunctiva. 

Mydriasis.  Forced  dilatation  of  the  pupil  by  means  of  drugs 
which  paralyze  the  concentric  muscles  of  the  iris  and  the 
ciliary.  The  two  drugs  commonly  used  for  this  purpose  are 
atropine  and  homatropine.  The  first  produces  a  profound  and 
lasting  mydriasis,  and  is  therefore  used  only  when  we  desire 
that  kind  of  dilatation.  The  latter  is  superficial  and  transitory 
in  action,  and  is  therefore  the  preferable  mydriatic  for  diagnosis 
or  refraction. 

Mydriatic.    A  drug  which  dilates  the  pupil. 

Myiocephalon.     Protrusion  of  the  iris  through  the  cornea. 

Myitis.  Mysitis.     Inflammation  of  the  muscles. 

Myodesopia.    Same  as  Muscae  Volitantes. 

Myopia.  That  condition  of  refraction  in  which  the  posterior 
principal  focus  of  the  eye  lies  in  front  of  the  retinal  plane,  so 
that  neutral  light  waves,  instead  of  focussing  on  the  retina, 
come  to  a  focus  before  they  reach  it,  are  reversed,  and  fall  on 
the  retina  in  diffusion  circles  of  plus  waves.  In  other  words, 
the  focal  length  of  the  refracting  system  of  the  eye  is  less 
than  the  antero-posterior  diameter  of  the  eye-ball. 

In  both  its  mathematical  and  its  clinical  aspects,  myopia  is 
the  simplest  of  all  the  forms  of  ametropia.  The  light  wave 
that  is  normal  to  a  myopic  eye,  i.  e..  which  will  focus  on  the 
retina  of  a  myopic  eye  at  rest,  is  a  divergent  wave,  having  its 
origin  inside  of  infinity ;  which  is  equivalent  to  saying  that  the 
far  point  of  a  myopic  eye  lies  within  infinity.  And,  as  this  is 
a  definite,  demonstrable  point  in  space,  the  myopic  far  point 
can  be  mathematically  expressed  and  clinically  measured  with- 
out any  trouble. 

Thus,  if  we  assume  an  eye  to  be  2  D.  myopic,  such  an  eye  at 
rest  can  focus  on  its  retina  only  a  light  wave  which,  when  it 
enters  the  cornea,  has  a  plus  curvature  of  2  D.  Such  a  wave 
will  have  a  radius  of  50  cm.,  measured  from  its  actual  point  of 


266  MYOPIA 

origin  anterior  to  the  cornea.     The  far  point  of  this  eye  is, 
therefore,  50  cm.  from  the  surface  of  the  cornea. 

THE  FAR  POINT. 

CHnically  the  myopic  far  point  is  determined  hy  the  same 
method  as  that  of  the  hyperope ;  that  is  to  say,  subjectively  by 
means  of  the  Snellen  chart,  objectively  by  means  of  static 
retinoscopy. 

The  myope's  far  point  being  inside  of  infinity,  he  cannot 
read  20/20  by  any  means  at  his  command.  He  reads  only  the 
type  whose  minimum  visual  angle  at  6  meters  or  20  feet  cor- 
responds to  the  angular  aperture  represented  by  his  far  point; 
and  the  number  of  this  type  as  a  denominator,  with  the  normal 
number  as  a  numerator,  expresses,  in  the  form  of  a  fraction, 
his  distant  vision.  If  he  reads  at  6  meters  the  type  which  an 
emmetrope  reads  at  8  meters,  his  vision  is  6/8,  or  3  4,  or  if  he 
reads  at  20  feet  the  type  an  emmetrope  can  read  at  30  feet, 
his  vision  is  said  to  be  20/30. 

To  enable  a  myope  whose  vision,  let  us  say,  is  20  40,  to  read 
the  20  type  at  20  feet,  we  must  make  the  20  type  subtend  a 
visual  angle  at  his  nodal  point  equal  to  the  angle  which  the  40 
type  now  subtends.  Translating  this  into  refraction,  this  means 
that  we  must,  by  means  of  minus  lens  power,  diverge  the  neu- 
tral light  waves  proceeding  from  the  chart  so  that  they  fall 
upon  his  cornea  with  a  plus  curvature  normal  to  his  eye.  i.  e., 
equal  to  the  reciprocal  of  his  far  point.  The  lens  which  does 
this  is  the  measure  of  his  myopia;  or,  what  is  the  same  thing, 
his  far  point  divided  into  unity  is  the  measure  of  his  myopia  in 
dioptres. 

In  static  retinoscopy,  with  a  plus  lens  before  the  patient's 
eye,  we  shall  always  find  the  myope's  point  of  reversal  nearer 
to  the  eye  than  the  focal  length  of  the  working  lens.  The 
calculation  is  made  in  the  same  manner  as  in  hyperopia,  i.  c., 
the  difference  between  the  place  where  it  ought  to  be  and  the 
place  where  it  is,  in  terms  of  reciprocals,  gives  us,  also  in  terms 
of  reciprocals,  the  i)atient's  far  point.  In  the  case  oi  the  myope, 
since  the  latter  (|uantity  is  always  greater  than  the  ft)riner,  the 
result  is  a  negatixc  i|uaiUity.  indicating;  th.it  llu'  pt»int  is  short 
of  infinitv. 


MYOPIA  267 

Thus,  if  with  a  plus  2  lens  we  find  the  point  of  reversal  at 
25  cm.,  whereas  it  ought  to  be  at  50  cm.,  then : 
1  1  —1 


.50  .25  .50 

which  is  to  say,  the  patient's  far  point  is  at  50  cm.,  (between 
the  eye  and  infinity),  and  he  is  2  D.  myopia. 

THE  NEAR  POINT. 
The  myope's  near  point  is  determined  by  precisely  the  same 
means  as  that  of  the  emmetrope  or  hyperope,  namely,  by 
Scheiner's  test,  by  Jarger  type,  or  by  dynamic  skiascopy.  It 
is,  of  course,  much  further  in  than  that  of  the  emmetrope,  cor- 
responding to  the  nearness  of  his  far  point.  It  is,  in  fact,  usually 
so  much  nearer  to  the  patient's  eyes  than  he  has  any  occasion 
to  accommodate  that  it  has  no  particular  working  value  in 
itself,  being  useful  only  for  purposes  of  calculation. 

AMPLITUDE  OF  ACCOMMODATION. 

According  to  the  formula  previously  given,  the  subtraction 
of  the  far  point  from  the  near  point,  in  terms  of  dioptrism, 
gives  us  the  myope's  amplitude  of  accommodation.  Myopes 
of  moderate  degrees  of  error  usually  have  fairly  normal  ampli- 
tude of  accommodation ;  and,  as  they  do  not  have  to  use  any 
of  their  accommodation  until  well  within  infinity,  they  possess 
a  greater  amount  of  available  accommodation  than  the  emme- 
trope by  just  the  amount  of  their  myopia.  Thus,  if  a  myope's 
far  point  be  at  50  cm.,  and  his  near  point  for  ordinary  work  at 
25  cm.,  he  uses  only 

1  1  1 


.25  .50  .50 

that  is,  2  D.  of  accommodation  for  this  working  point,  whereas 
the  emmetrope  must  use 

1  1  1 


.25  inf.  .25 

that  is  to  say,  4  D.  for  the  same  point,  giving  the  myope  an 
advantage  of  2  D.  available  accommodation. 


268  MYOPIA 

ACCOMMODATION-CONVERGENCE  RELATION. 

Mathematical!}",  myopia  carries  with  it  precisely  the  same 
proportional  amount  of  accommodation-convergence  discrep- 
ancy, dioptre  for  dioptre,  as  hyperopia ;  for,  while  every  dioptre 
of  hyperopia  represents  excess  accommodation,  every  dioptre 
of  myopia  represents  the  same  amount  of  excess  convergence. 
Physiologically,  however,  the  state  of  affairs  is  altogether  dif- 
ferent. The  abnormal  relation  in  myopia  is,  so  to  speak,  a 
negative  or  passive  condition.  'Jhe  two  functions  do  not  ac- 
tively clash,  as  in  hyperopia;  one  of  them  simply  '"lays  off"." 
Lacking  accommodation  stimulus  to  bring  his  convergence 
into  play,  outside  his  far  point,  the  myope  simply  replaces  it 
altogether  with  fusion  stimulus;  inside  his  near  point  he  makes 
up,  with  fusion  stimulus,  the  deficit  in  his  accommodation 
stimulus.  As  he  uses  neither  accommodation  nor  convergence 
at  infinity,  there  is  nothing  to  cause  a  development  of  muscular 
imbalance  at  that  point.  This  whole  matter  will  be  found  fully 
discussed  in  the  chapter  on  Heterophoria. 

When  we  come  to  consider  the  state  of  aff'airs  at  the  myope's 
near  point,  the  same  thing  holds  true.  Myopes  of  moderate 
degrees  usually  have  fairly  normal  amplitude  of  accommoda- 
tion ;  and,  as  they  do  not  begin  to  use  their  accommodation 
until  well  within  infinity,  they  have,  as  a  rule,  more  available 
accommodation  than  the  emmetrope  by  just  the  amount  of 
their  myopia.  Add  to  this  the  fact  that  the  point  at  which 
the  average  myope  does  his  near  work  is  far  from  being  his 
maximum  near  point,  and  it  will  reailily  be  seen  that  lu-  has 
plenty  of  accommodati\e  reserve. 

For  these  reasons,  in  spite  of  the  break  in  his  accotninoda- 
tion-convergence  ratio,  the  average  myope  has  none  of  the 
troubles,  either  at  far  point  or  at  near  point,  that  plague  the 
hyperope ;  he  seldom  suffers  from  any  muscular  asthenopic 
symptoms;  nor  do  we  encounter  the  sources  of  erritr  in  re- 
fracting a  myoi)e  that  we  meet  in  refracting  the  hyperope.  The 
correction  of  his  myopia  establishes  normal  relations  between 
accommodation  and  convergence, — which  may  bother  him  for 
a  little  while,  but  to  which  he  soon  adjusts  himself,  since  his 
reserve  and  flexibility  are  ample.     An  exception  must  be  made 


MYOPIA  269 

in  the  case  of  high  myopes,  concerning  whom  more  will  be 
said  presently. 

SUMMARY. 

The  course  of  procedure  in  examining  and  prescribing  for 
average  degree  myopes,  therefore,  is  exceedingly  straightfor- 
ward and  simple.  Ordinarily,  all  that  is  required  is  to  find 
their  far  point  for  each  eye,  which  is  equivalent  to  determining 
their  myopic  error  for  distance,  and  to  furnish  them  with 
distance  correction  for  constant  wear. 

1.  Make  a  subjective  test  with  the  Snellen  chart  at  20  feet, 
adding  successively  stronger  minus  lenses  until  the  patient 
reads  20/20.  As  the  patient  is  already  "in  the  fog,"  when  the 
test  is  begun,  on  account  of  his  own  myopia,  this  test  amounts 
to  an  application  of  the  fogging  method. 

Special  care  should  be  taken  to  include  the  wheel  chart  in 
the  procedure,  to  discover  any  possible  astigmatism  ;  for  most 
myopes  are  also  astigmatic. 

2.  Determine  the  refraction  by  static  retinoscopy. 

3.  Make  a  test  for  muscle  imbalance  at  near  point. 

This  muscle  test  will  usually  disclose  considerably  more 
exophoria  than  in  the  emmetrope ;  but  this  is  because  most  of 
the  myope's  convergence  is  fusion  convergence,  and  is  readily 
surrendered  under  the  test.  Unless,  therefore,  it  exceeds  12 
prism  dioptres  at  25  cm.,  or  the  patient  is  complaining  of 
asthenopia,  it  need  not  be  regarded.  If  it  exceeds  12  prism 
dioptres,  or  if  it  gives  definite  reading  trouble,  it  had  better  be 
corrected  with  prisms,  base  in,  up  to  a  point  of  comfort. 

In  the  average  case  of  moderate  degree  myopia  it  is  not 
necessary  to  explore  the  near  point  at  all.  If  the  myopic  patient 
is  also  presbyopic,  his  presbyopia  must,  of  course,  be  investi- 
gated and  corrected  as  in  the  case  of  any  other  presbyope  (see 
Presbyopia).  If  he  show  symptoms  of  accommodative  ineffi- 
ciency, this  must  be  explored  by  the  same  means  and  according 
to  the  same  rules  laid  down  in  Hyperopia. 

HIGH   MYOPIA.     PROGRESSIVE  MYOPIA. 

Most  of  the  troubles  which  we  encounter  in  dealing  with 
myopes  are  furnished  by  cases  of  high  myopia.  Practically  all 
of  these  cases  represent  a  pathological  condition  of  the  eye. 


270  MYOPIA 

Indeed,  some  regard  myopia  as  being  per  se  a  pathological 
state,  representing  the  attempt  of  nature  to  adapt  the  eye  to 
near  vision  required  by  the  exigencies  of  civilized  life.  The 
author  does  not  share  this  view ;  but  it  is,  of  course,  beyond 
question  that  the  physical  conditions  attending  myopia — the 
elongated  eyeball,  the  tense  chorioid,  the  necessity  for  poring 
over  one's  work — are  such  as  may  very  easily  cross  the  border- 
line from  functional  to  organic  trouble.  And  in  almost  every 
case  of  high  myopia,  the  line  has  been  crossed. 

The  most  serious  feature  of  this  pathology  is  the  damage 
done  to  the  chorioid.  The  ophthalmoscope  shows  a  crescentic 
white  patch  of  atrophy  on  the  macular  side  of  the  disc,  where 
the  greatest  amount  of  stretching  takes  place ;  and  often  it 
reveals,  also,  patches  of  chorioidal  degeneration.  In  very  high 
degrees  of  myopia  the  epithelial  layer  of  the  retina  atrophies 
and  secondary  changes  may  occur  in  the  macula  itself.  .Ml  oi 
these  diseased  conditions  are  prone  to  increase  year  by  year — 
sometimes  even  from  month  to  month — constituting  what  is 
known  as  "progressive  myopia,"  so  that,  ultimately,  the 
\itreous  may  become  disorganized,  cataracts  form,  chorioidal 
hemorrhages  occur,  and  retinal  detachment  take  place. 

From  an  optical  standpoint,  the  chief  difficulties  in  the  way 
of  satisfactorily  correcting  high-degree  myopes  are: 

1.  In  extreme  degrees  of  myopia  the  high-power  minus 
lenses  necessary  for  full  distance  correction  usually  reduce  the 
size  of  the  image  on  the  retina  to  such  an  extent  that  the  pa- 
tient is  unable  to  recognize  objects.  It  is,  tiierefore.  imjjrac- 
ticable,  as  a  rule,  to  give  these  patients  lull  (.orn-ctioii  iov  dis- 
tance ;  we  are  obliged  to  guide  ourselves  by  the  anutuut  of 
correction  which  gi\es  the  best  possible  vision. 

2.  The  ciliary  muscles,  sharing  in  the  general  p(»i)r  nutri- 
tion of  the  eye,  and  by  reason  of  disuse,  are  greatly  atrophied  ; 
often  the  circular  fibres  are  lacking  altogether.  .Xs  a  result, 
the  patient  has  no  accommodative  power,  so  that,  if  lull  dis- 
tance correction  be  gi\en,  or  even  sufiicieut  distance  correction 
to  bring  the  far  \n)'u\i  outside  (»f  tlu-  patient's  reading  distance, 
he  is  unable  to  si-e  at  that  reading  distance,  lie  is,  in  fact,  a 
presbyope  with  his  distance  correction  on.  l'"or  this  reason,  it 
is  necessary  to  give  such  patients  separate  glasses  for  distant 


MYOPIC  CRESCENT  271 

and  near  vision,  subtracting  from  the  distant  lenses  the  recip- 
rocal of  the  distance  at  which  the  patient  wishes  to  read  and 
prescribing  the  remainder  for  reading  glasses. 

3.  The  elongation  of  the  eyeball,  with  its  consequent  equa- 
torial distortion,  makes  the  action  of  the  internal  rectus  muscles 
in  convergence  exceedingly  difificult.  This  embarrassment 
usually  results  in  one  of  three  eventualities : 

(a)  The  patient  continues  to  endure  the  situation,  and  to 
perform  his  convergence  under  difficulty,  suffering  meanwhile 
from  eyestrain,  until  exophoria  results  in  divergent  squint,  or, 

(b)  He  substitutes  for  convergence  the  practice  of  making 
a  conjugate  turn  of  the  two  eyes,  which  also  produces  at  length 
a  divergent  strabismus,  or, 

(c)  He  avoids  convergence  by  using  one  eye  only  for  his 
near  work,  and  as  he  usually  selects  one  eye  (the  dominant 
eye)  for  continuous  duty,  the  other,  unused  eye  ultimately  be- 
comes amblyopic. 

This  muscular  trouble,  with  its  train  of  mischievous  conse- 
quences, is  best  remedied,  and  the  consequences  headed  ofif.'by 
the  intelligent  application  of  prisms,  base  in,  for  near  work,  up 
to  the  point  of  comfort  and  efficiency. 

Myopic  Crescent.  A  crescent-shaped  area  of  white  seen  at  the 
temporal  side  of  the  optic  disc  in  high  myopia.  It  is  caused 
by  the  stretching  and  thinning  of  the  chorioid,  permitting  the 
sclera  to  show  through  and  reflect  a  white  light  from  the 
ophthalmoscope. 

Myosis  and  Myotic.    Same  as  Miosis  and  Miotic. 

Myospasm.     Spasm  of  a  muscle. 

Myotomy.    The  cutting  or  dividing  of  a  muscle. 

Myotonic.     Pertaining  to  muscle  spasm. 

Nagel's  Test.  A  test  for  color-blindness  with  confusion  colors 
printed  in  concentric  circles. 

Nasal  Duct.  That  part  of  the  tear  duct  which  passes  through 
the  malar  bone  and  into  the  nose. 


272  NEAR  POINT 

Near  Point.  The  nearest  point  for  which  the  eyes  are  able  to 
accommodate  (near  point  of  accommodation)  or  converge  (near 
point  of  convergence),    .^ee  Accommodation  and  Convergence. 

Near  Sight.     A  collocjuial  term  lor  myopia. 

Nebula.     Tiny  scattered  opacities  on  the  cornea. 

Needling.  An  operation  for  soft  cataract.  The  needle  is  intro- 
duced through  the  cornea,  and  the  anterior  surface  of  the  lens 
pierced  in  several  places,  or  torn  by  the  needle.  The  action  of 
the  aqueous  humor  then  causes  the  cataract  to  swell  up  and  be 
absorl)ed. 

Negative,  in  ph)sical.  including  optical,  science,  this  term  is 
applied  to  a  phenomenon,  or  a  quantity,  which  occurs,  or  has 
its  value,  in  a  direction  opposite  to  that  of  the  phenomenon 
or  quantity  toward  which  it  is  said  to  be  negati\  e. 

Negative  .Accommodation.  It  is  supposed  by  some  that 
when  the  ciliary  muscle  is  adajjted  for  infinity  it  is  still  capable 
of  a  slight  further  relaxation.  This  additional  amount  of  re- 
laxation is  known  as  negative  accommodation. 

Negative  .After-images.  After-images  which  are  the  result 
of  retinal  exhaustion  by  the  primary  image. 

Negative  Convergence.  Turning  of  the  \  isual  axes  outward 
beyond  the  parallel. 

Negative  Crystals.  Cry.stals  of  double  refracting  properly 
in  w  hich  the  refraction  of  the  ordinary  ray  is  greater  than  that 
of  the  extraordinary  ray. 

Ncgatixe  Focus.  .\n  imaginar}'  focus  arrixcd  at  by  project- 
ing divergent  waves  back  to  their  apparent  point  of  origin. 

Negative  Image.  The  \  irtual  image  made  by  negative 
focussing. 

Neotocophthalmia.     Sit-  Ophthalmia  Neonatorum. 

Nephablepsia.     .'^iiow  -bliiulness. 

Nephelopia.     t  lomly  vision. 

Nephritic  Retinitis.  Same  a-  Albuminuric  Retinitis,  .^ee  Retin- 
itis and  Ophthalmoscope. 


NEURASTHENIA,  OPTIC  273 

Neurasthenia,  Optic.  A  nervous  condition  in  which  the  visual 
field  is  narrowed. 

Neuritis,  Optic.  Inflammation  of  the  optic  nerve.  When  the 
part  of  the  nerve  back  of  the  eyeball  is  involved  it  is  called 
retro-bulbar  neuritis.  Under  the  ophthalmoscope  it  shows  as 
a  reddened,  congested  condition  of  the  nerve-head  (disc). 

Neurochorioiditis.  Inflamed  condition  of  the  chorioid  coat  of 
the  eye  and  the  optic  nerve. 

Neurochorioretinitis.  Inflamed  condition  of  the  chorioid  coat  of 
the  eye,  the  retina  and  the  optic  nerve. 

Neurodealgia.     Excessive  sensibility  of  the  retina. 

Neurodeatropia.    Atrophy  of  the  retina. 

Neuroretinitis.  Inflammation  of  both  the  optic  nerve  and  the 
retina.  Both  the  disc  and  the  surrounding  retina  are  reddened 
and  congested,  with  minute  hemorrhages  into  the  retina. 

Neurospongium.    Collection  of  fibrils  supporting  the  neuroplasm. 

Neutral.  In  optics  this  term  is  applied  to  light  waves  which 
have  no  curvature,  and  whose  rays  of  direction  are  parallel. 
They  are  also  called  parallel  waves.  As  they  appear  to  have 
come  from  no  point,  and  to  be  going  to  no  focus,  they  are 
further  known  as  infinite  waves. 

Neutralize.  To  render  a  plus  or  a  minus  light  waxe  neutral,  i.  e., 
to  rob  it  of  its  plus  or  minus  curvature,  so  that  it  appears  to 
have  come  from  infinity. 

The  word  also  technically  signifies  to  determine  the  dioptric 
power  of  a  lens  by  placing  in  apposition  with  it  a  lens  of  the 
opposite  curvature  which  neutralizes  the  parallax  movement. 
(See  Lens).  In  neutralizing  with  the  trial  case  there  is  always 
a  slight  amount  of  error,  due  to  the  impossibility  of  obtaining 
exact  apposition  of  the  surfaces.  The  convex  neutralizing 
lens  should  always  be  held  next  to  the  observer's  eye. 

Nictitition.     Involuntary  twitching  of  the  eyelids. 

Night  Blindness.     See  Blindness. 


274  NIPHABLEPSIA 

Niphablepsia.     Same  as  Nephablepsia. 

Nodal  Points.    Two  points  in  a  lens,  or  lens  system,  on  the  prin- 
cipal axis,  so  situated  that  oblique  rays,  which  enter  and 

emerge  from  points  whose  tangents  are  parallel  to  each  other, 

directed  toward  one,  will  appear  to  come  from  the  other,  after 

lateral  displacement.     (See  Lens). 

Mathematically,  the  nodal  points  are  two  conjugate  points 

from  which  object  and  image  appear  under  the  same  angle. 

Normal.  Physiologically,  this  word  signifies  conformance  to  the 
natural  order  of  things.  Optically,  it  is  applied  to  a  straight 
line  which  strikes  a  curved  surface  perpendicularly  to  its  tang- 
ent. 

Nubecula.    Cloudiness  of  the  cornea. 

Nuclear.    Pertaining  to  the  centre  of  an  organ. 

Nuclear  cataract.  One  which  begins  in  the  centre,  or  nu- 
cleus, of  the  crystalline  lens. 

Numeration  of  Lenses.    See  Lens. 

Nyctalopia.     Night  l)lindness. 

Nyctotyphlosis.     Blindness  at  night-time. 

Nystagmograph.  An  instrument  for  registering  the  movements 
of  the  cyeljall  in  nystagmus. 

Nystagmus.  Rapid,  involuntary  oscillations  of  the  eyel^all. 
Usually  due  to  central  nervous  disease.  It  is  termed  vertical, 
lateral,  or  rotary  nystagmus,  according  to  the  direction  of  the 
oscillations. 

Aural  Nystagmus.  Spasmodic  nystagmus  due  to  disturb- 
ances in  the  internal  ear. 

Miner's  Nystagmus.  Nystagmus  occurring  in  coal  miners, 
due  to  wielding  a  pick  while  lying  on  the  side  in  a  cramped 
position.  When  this  nystagmus  occurs  upon  turning  the  eyes 
downward,  it  is  called  miners'  nystagmus  against  the  rule. 

Labyrinthine  Nystagmus.     .Same  as  aural  nystagmus. 

Obcecation.     I'arlial  blindness. 


OBFUSCATION  275 

Obfuscation.    Obscuring  of  the  vision. 

Object.  In  optics  this  name  is  given  to  a  body  or  area  from 
which  waves  of  light  originate  that  are  focussed  upon  the 
retina  for  the  purpose  of  vision.  The  object  and  the  image 
are  conjugate  with  each  other.     (See  Lens). 

Object  Blindness.     Mind  blindness. 

Object  Line.  A  straight  line  in  the  object-space  containing  the 
optical  axis  of  a  lens-system.  With  reference  to  the  eye,  a 
straight  line  between  the  nodal  point  and  the  object,  con- 
necting the  object  with  the  macula. 

Object  Plane.  The  plane,  perpendicular  to  the  axis,  in  which 
are  situated  the  constituent  points  of  an  object  in  a  lens 
system. 

Object  Point.  Any  one  of  the  constituent  points  in  the  object 
plane  as  described  above. 

Object  Space.  The  space  traversed  by  the  effective  rays  orig- 
inating in  the  constituent  points  of  the  object  in  a  lens  system. 
See  Collinear  Space  System. 

Object  Test-Card.  A  test  chart  for  illerates  and  children,  with 
objects  instead  of  letters. 

Objective.  Physiologically,  this  term  signifies  a  phenomenon,  or 
set  of  phenomena,  which  are  viewed  and  considered  as  being 
outside  of  and  separate  from  one's  own  body,  as  distinct  from 
subjective  phenomena,  which  form  a  part  of  one's  own  sensa- 
tions. 

Objective  symptoms.  Symptoms  which  the  observer  dem- 
onstrates, as  distinct  from  those  felt  by  the  patient. 

Objective  tests.  Tests  which  depend  upon  the  observer's 
findings,  apart  from  the  patient's  information,  such  as  retino- 
scopy. 

Optically,  the  word  objective  is  applied  to  a  reflecting  or 
refracting  system  whose  purpose  is  to  form  an  image  of  an 
object,  which  image  is,  in  its  turn,  to  serve  as  an  object  for 
further  focal  imaging. 


276  OBLIQUE 

Oblique.     Making  an  angle  with  the  perpendicular. 

(Jblique  astigmatism.  Astigmatism  in  which  the  chief 
meridians  are  neither  vertical  nor  horizontal. 

Oblique  axis.    An  axis  off  the  jjerpendicular. 

Oblique  illumination.  Illumination  of  an  object  on  one 
side,  by  passing  the  light  obliquely  through  a  lens. 

Oblique  Muscles.  The  muscles  which  move  the  eyeball 
obliquely. 

Occipital.  Pertaining  to  tlie  back  part  of  the  head  and  the 
muscles  which  are  attached  to  it.  These  muscles  are  usually 
fatigued  in  astigmatic  patients,  giving  an  occipital  headache. 

Occipito-Frontalis.  The  muscle,  originating  in  the  occiput  and 
inserted  into  the  fascia  of  the  eyebrows,  which  lifts  them 
upward. 

Occlusion.  Blocking  the  pu])il  l)y  a  membrane,  as  in  iri(U»c\cli- 
tis. 

Ocellus.     Having  a  single  eye. 

Ocular.     Pertaining  to  the  eye. 

Ocular  Spectres.     See  Muscae  Volitantes. 

Oculist.  A  medical  practitioner  who  specializes  in  disorders 
and  diseases  of  the  eye. 

Oculo-Motor.  A  term  applied  to  the  third  cranial  nerve  because 
it  mediates  movements  of  the  eye. 

O.  D.     .\bbre\iation  for  Oculus   Dexter,  the  right  eye. 

Offset  Guard.  A  guard  on  an  eyeglass  for  the  purpose  of  hold- 
ing the  lens  further  from  the  eye.     See  Spectacles. 

Old  Sight.     A  collo(|uial  term  for  presbyo|)i;i. 

Onyx.     I 'us  between  the  layers  of  the  cornea. 

Opacity.  Imperviousness  to  tiie  passage  of  light.  (Opacities  of 
the  eye  ;ire  practically  always  either  corneal  or  lenticular. 

Opaque.     Impervious  to  the  jiassage  of  light. 


OPERCULUM  OCULI  277 

Operculum  Oculi.    The  eyelid. 

Ophryitis.     Inflammation  of  the  eyebrows. 

Ophrys.     The  eyebrow. 

Ophthalmagra.     Sudden  pain  in  the  eyeball. 

Ophthalmalgia.    Same  as  above. 

Ophthalmia.    Severe  infection  of  the  entire  eye. 

Ophthalmia  Neonatorum.  A  form  of  purulent  conjunctivitis 
which  attacks  newborn  children,  due  to  infection,  usually  with 
gonorrheal  germs,  during  their  passage  through  the  birth- 
canal.  In  order  to  guard  against  its  occurrence,  most  States 
have  enacted  a  law  requiring  the  attending  surgeon  or  mid- 
wife to  instill  into  every  baby's  eyes  at  birth  a  weak  solution 
of  silver  nitrate. 

Ophthalmitis.     Same  as  Ophthalmia. 

Ophthalmoblennorhea.  Profuse  flow  of  pus  from  the  conjunc- 
tiva. 

Ophthalmo-Carcinoma.     Cancer  of  the  eye. 

Ophthalmocele.     See  Stapyloma. 

Ophthalmocopia.     Fatigue  of  the  eyes. 

Ophthalmodynia.     Neuralgic  pain  in  the  eye. 

Ophthalmography.     Descriptive  lore  of  the  eye. 

Ophthalmologist.  One  who  is  learned  in  the  science  of  the  eye 
and  all  that  pertains  to  it.  The  word  is  commonly  used  inter- 
changeably with  "oculist,"  but  it  is  really  a  much  broader 
term.    There  are  many  ophthalmologists  who  are  not  oculists. 

Ophthalmology.  The  science  of  the  eye  and  all  that  pertains 
to  it. 

Ophthalmomacrosis.     Enlargement  of  the  eyeball. 

Ophthalmomalacia.    Abnormal  softness  of  the  eyeball. 


278  OPHTHALMOMETRY 

Ophthalmometry. 

'The  ophthahiiometer  is,  without  doubt,  a  much  abused 
and  consequently  a  much  disused  instrument.  It  i^  prob- 
ably true  that  a  discussion  of  ophthalmometers  and  ophthal- 
mometry in  a  group  of  men  interested  in  refraction  would 
evoke  \ery  decided  expressions  of  opinion  both  favorable 
and  unfavorable  to  its  usefulness.  Be  that  as  it  may,  we  are 
of  the  opinion  that  the  ophthalmometer  is  one  of  the  most 
valuable  instruments  that  we  have  in  refractive  work  and 
we  hope  to  defend  our  position  in  the  succeeding  paragraphs. 
Incidentally,  let  it  be  stated  at  once  that  the  findings  by  oph- 
thalmometry and  the  ultimate  prescription  given  the  patient, 
in  so  far  as  the  astigmatic  correction  is  concerned,  may  not  be 
in  agreement.  There  are  various  reasons  why  this  may  occur. 
Furthermore,  differences  in  various  sets  of  findings  by  dif- 
ferent methods  (e.  g.  ophthalmometry,  retinoscopy — both 
static  and  dynamic — and  subjective  tests — both  with  and 
without  cycloplegics)  should,  lead  to  a  careful,  .analytical 
study  of  the  case  in  hand  from  all  standpoints.  X'ariations  in 
data  by  different  methods  ought  always  to  furnish  us  with 
the  information  needed  for  final  analysis  of  the  case  and  the 
expression  of  our  judgment  as  to  the  corrections  to  be  offered 
or  the  treatment  to  be  accorded.  Let  us,  then,  briefly  state 
the  following  facts  germane  to  the  topic  under  discussion : 
(1)  The  ophthalmometer  measures  anterior  corneal  curva- 
tures and  astigmatic  conditions  only  ;  hence  these  findings  may 
be  ultimately  modified  l)v  posterior  corneal  conditions  or  by 
lenticular  conditions  iiuolving  regular  or  irregular  astigma- 
tism. (2)  The  subjective  findings  and  corrections  are  obtained 
with  lenses  inserted  somewhere  near  the  region  of  the  anterior 
focus  of  the  eye:  the  effectivity  of  all  lenticular  corrections 
varies  with  the  distances  from  the  eye  at  which  they  arc  placed  : 
hence  ophthalmometric  findings,  on  the  one  hand,  and  sub- 
jective and  relinoscopic  data  on  the  other  hand,  may  not 
apijarently  agree,  whereas  they  may  be  in  very  good  accord 
when  i)roper  allowance  is  made  for  the  conditions  which  have 

•Ol'HTHAIJVlOMKTUY  AND  ITS  AI'l'I JCATION  TO  OCl'l.AU  UKKUAC- 
TION  AM)  KYIO  KXAMINATIONS.  »>y  Ouirhs  Shcurcl.  Ph.  D..  rh.vHloloKloiil 
opllclHt.  Tho  AintTlcan  ()i)tl<iil  C'oinpuny,  likJItor  of  The  Amerlriin  Joiiriuil  of 
l'hy.slologlc'ttl  OpllcH  «'t  ul. 

Copyright,   IKliO.  Anu-rloun  Optical  Ci)inpany. 


OPHTHALMOMETRY 


279 


just  been  cited.  (3)  The  ophthalmometer  does  accurately 
measure  the  curvatures  in  various  meridians  of  the  anterior 
corneal  portion  of  the  eyeball,  hence  affords  some  evidence 
as  to  whether  or  not  the  myopia  or  hyperopia  subsequently 
disclosed  by  other  tests  is  due,  possibly,  to  axial  or  refractive 
conditions.  (4)  It  furnishes  a  ready  means  of  discovering 
irregularities  of  the  cornea,  which  cause  irregular  astigmatism 
and  generally  produce  reductions  in  the  visual  acuities. 

THE  OPHTHALMOMETER. 

The  first  instrument  was  devised  by  von  Helmholtz,  who 
published  a  description  in  1854.  In  the  80's  Javal  and  Schiotz 
improved  the  instrument  and  gave  it  a  semblance  of  the  form 
it  has  today.  Thus  far  ophthalmometers  measuring  corneal 
curvatures  only  have  been  perfected.  Claims  have  been  made 
as  to  the  invention  of  instruments  measuring  lenticular  condi- 


Ba. 


Images 


Figure  1.  Diagram  illustrating  the  essential 
principles  underlying  the  construction  and  use  of 
the  ophthalmometer.  The  four  images  as  pro- 
duced by  the  doubling  device  are  shown  in  an 
enlarged    form   at   the   lower   right-hand    corner. 


280  OPHTHALMOMETRY 

tions,  but  none  have,  to  our  knowledge,  been  sufficiently  per- 
fected to  warrant  their  general  use  in  office  practice.  It  is 
obvious  that  if  the  prisms  in  the  telescope  of  the  ophthalmo- 
meter are  made  movable,  so  that  they  can  be  shoved  to  and 
from  the  eye,  we  should  have  a  scheme  which,  theoretically  at 
least,  should  enable  us  to  examine  the  cornea  and  the  anterior 
and  posterior  surfaces  of  the  lens.  Special  mires  would  be 
needed  in  order  to  give  sufficient  illumination  and  clarity  of 
images.  Such  an  instrument  ought,  if  perfected,  to  aid  in  the 
settlement  of  many  points  of  scientific  interest;  sivch,  for 
example,  as  the  mechanism  of  accommodation,  the  (|uestion 
of  accommodative  astigmatism,  et  al. 

The  fundamental  parts  of  the  ophthalmometer  are  the  tele- 
scope, which  carries  the  ol)jective  and  an  eye  lens,  a  double 
refracting  prism  and  a  set  of  mires.  In  some  instruments  the 
overlapping  or  separation  of  images  of  the  mires  as  seen 
through  the  telescope,  is  controlled  by  a  mo\ement  of  the 
mires  through  the  rotation  of  a  worm  device  whereby  the 
mires  are  moved  along  an  arc  ;  in  another  type  of  instrument 
the  mires  are  stationary  but  the  positions  of  the  reflected 
images  of  the  mires,  as  seen  by  the  operator,  are  varied  by 
changes  in  the  position  of  the  prisms.  These  prisms  are  fitted 
into  a  special  tube  and  carried  back  and  forth  by  a  rack  and 
pinion  device.  In  certain  instruments  the  so-called  "primary" 
and  "secondary"  positions  can  be  readily  turned  from  one  to 
the  other  lespectively  by  a  simple  rotation  of  the  prisms.  One 
form  of  instrument,  not  as  successfully  developed  as  we  judge 
it  can  and  ought  to  be,  gives  both  primary  and  secondary 
findings  in  the  field  simultaneously. 

The  birefracting  prism  is  of  quartz  and  is  so  cut  with  re- 
spect to  its  optic  axes  as  to  give  double  images,  ihis  prism 
is,  in  reality,  composed  of  two  prisms  with  the  apices  in  oppo- 
site directions,  so  placed  as  to  cause  the  deviation  to  take 
place  from  each  side.  Since  each  prism  produces  a  certain 
degree  of  deviation,  twice  the  amount  of  separation  is  secured 
and  at  the  same  time  the  doul)le<l  images  can  be  kept  nearer 
the  center  of  the  field.  The  prisms  are  so  mounted  that  their 
plane  of  doubling  is  in  exact  line  with  the  plane  oi  ilu-  grad- 
uated arc. 


OPHTHALMOMETRY 


281 


The  telescope  should  be  equipped  with  combinations  of 
achromatic  lenses.  This  is,  in  general,  approximately  the  case, 
but  we  find  in  practice  that  the  reflected  images  of  the  mires 
as  seen  in  the  field  of  vision  under  the  magnification  of  the 
ocular  of  the  telescope  are  often  accompanied  with  colored 
fringes  and  indistinct  edges  which  interfere  with  the  deter- 
mination of  the  exact  coincidence  of  the  mires  in  corneal  exam- 
inations. These  slight  color  fringes  are  due,  without  doubt, 
to  diffraction  effects  which  are  produced  by  the  texture  of 
the  cornea  and  which  may,  for  example,  cause  the  general 
character  of  the  light  reflected  from  the  cornea  as  seen  in  the 
telescope  to  be  bluish  or  reddish ;  they  may,  as  their  second 
source,  be  due  to  the  dispersive  effects  of  the  prism  system 
producing  the  doubled  images;  or,  again,  to  lack  of  complete 
achromatism  of  the  telescopic  lens  systems. 


Figure    2. 


Detailed    diagram    of    the    objective,    prism    system,    and    ocular    of 
the    telescope    of   the    ophthalmometer. 


This  clearness  and  sharpness  of  the  edges  of  the  mires  and 
the  freedom  from  colored  edges  in  the  region  of  contact  of 
reflected  images  may  be  acquired,  so  E.  LeRoy  Ryer  says, 
through  the  use  of  a  red  screen  over  one  mire  and  a  green 


282  OPHTHALMOMETRY 

screen  over  the  other  mire,  these  screens  having  as  nearly  as 
possible  complementary  colors.  It  is  desirable,  however,  to 
have  access  to  white  mires  under  other  circumstances;  hence 
both  purposes  may  be  accomplished  by  obtaining  red  and 
green  colored  celluloid  pieces  which  may  be  slipped  in  between 
the  glass  plates  of  the  mires  and  the  mires  proper.  These 
may  be  inserted  or  removed  in  a  moment's  time.  The  writer 
has  tried  this  scheme  and  found  that  it  is  excellent  in  some 
cases.  For  his  own  part,  however,  he  has  to  state  that,  with 
oppositely  colored  mires,  the  phenomenon  of  different  foci  for 
different  colors  (i.  e.,  stereoscopic  vision  for  colors)  occurs, 
so  that  the  mire  with  the  red  screen  is  not  sharply  seen  when 
the  green  one  is.  But  the  region  of  overlapping  and  that  of 
"just  in  contact"  are  generally  sharply  defined.  The  eft'ects 
due  to  a  set  of  white  mires  and  a  set  consisting  of  a  red  and  a 
green  one  may  be  simultaneously  obtained  and  the  contrast 
noted  by  inserting  the  colored  celluloid  strips  in  such  a  way  as 
to  cover  only  half  of  each  mire  respectively.  We  note,  in  pass- 
ing, that  one  of  the  American  companies  manufacturing  oph- 
thalmometers has  for  some  years  stated  in  its  descriptive 
literature  that  the  mires  may  be  of  any  color  desired,  one  red 
and  the  other  green  and  so  on,  or  both  may  be  of  white.  It  is 
their  recommendation,  however,  that  the  mires  be  w^hite  as  a 
general  rule. 

SOME    PRACTICAL    POINTS    IN    THE    MANIPULATION    AND 
USE   OF   THE   OPHTHALMOMETER. 

From  the  conversations  we  have  had  with  many  practi- 
tioners and  from  the  questions  asked  or  criticisms  raised,  w  c 
judge  that  many  men  do  not  thoroughly  undirstand  the  nice- 
ties of  manipulation  of  an  ophthalmometer,  lii-nce  it  is  desir- 
al)le  that  attention  be  called  to  these  points. 

1.  The  eye-piece.  The  examiner  should  first  of  all  a(lju>t 
the  telescope  bv  looking  throuj^li  it  and  turninj;  the  eye-piece 
(the  end  nearest  the  eye)  until  the  cri>ss-hairs  are  in  perfrct 
focus.  'I'his  dilYers  for  in(li\  idual  operators,  hence  the  reason 
for  till-  ailjustnuMit.  Likewise,  these  cross-hairs  are  easil) 
gotten  out  of  focus  in  the  course  of  the  use  of  the  instrument. 
If  these   cross-hairs  are   not    in    proper   ft)cus.   the   images   re- 


OPHTHALMOMETRY  283 

fleeted  from  the  cornea  cannot  be  brought  into  correct  focus, 
hence  cannot  be  made  as  clearly  and  sharply  defined  as  they 
should  be.  This  adjustment  of  the  eye-piece  of  the  telescope 
is  a  frequent  omission  on  the  part  of  many  practitioners.  It 
should  be  stated,  in  passing,  that  some  makes  of  instruments 
do  not  have  such  a  device;  this  adjustment  would  not  then  be 
called  for. 

2.  The  patient  should  be  comfortably  seated  and  the  instru- 
ment placed  upon  an  adjustable  table  or  stand  in  order  that  a 
comfortable  and  uncramped  position  of  the  head  and  body  of 
the  patient  may  be  assumed.  Also,  the  operator  should  give 
some  heed  to  his  own  comfort  and  efficiency  in  his  work. 

3.  The  forehead  of  the  patient  should  be  pressed  against  the 
top  of  the  head-piece  and,  when  in  readiness  for  observations, 
the  patient  should  be  instructed  to  keep  the  head  immobile. 

4.  The  patient  should  be  instructed  to  look  at  the  end  of 
the  telescope.  If  necessary  a  small  white  fixation  object  may 
be  provided  and  attached  very  close  to  the  mouth  of  the  tele- 
scope. 

5.  The  line  of  sight  of  the  eye  and  telescope  may  be  approx- 
imated initially  by  observing  the  relative  heights  of  the  eye 
and  telescopic  tube  through  the  slits  or  apertures  provided  in 
the  metal  screen  to  which  is  attached  the  arc  carrying  the 
mires,  and  a  little  above  or  to  the  side  of  the  telescope.  Very 
little  further  adjustment  should  be  called  for  in  this  respect 
since  the  images  from  the  cornea  should  be  readily  obtained 
and  seen  by  the  observer  upon  swinging  the  telescope  around 
in  line  with  the  eye  under  test  and  by  adjusting  the  telescope 
back  and  forth  on  the  rack  and  pinion  device  provided  for  the 
purpose  of  focusing  the  images  of  the  mires. 

6.  The  images  of  the  mires  reflected  from  the  cornea  are  to 
be  focused  so  that  they  are  the  clearest  and  sharpest  possible 
by  a  movement  of  the  telescope  only,  without  touching  the 
eye-piece  if  such  is  present  and  in  adjustment.  Because  of 
backlash  it  is  advisable  to  always  finally  focus,  whether  in  the 
primary  or  secondary  positions,  from  the  same  direction ;  i.  e., 
always  finally  focusing  from  a  movement  of  the  telescope  for- 


284 


OPHTHALMOMETRY 


ward  or  vice  versa.  This  point  should  always  be  heeded  when 
comparing  the  results  or  data  on  the  same  cornea  obtained  by 
dirt'crent  makes  and  styles  of  instrument. 


Figure  3.  The  corneal  reflections 
as  obtained  with  one  form  of 
modern    ophthalmometer. 


Figure  4.  The  corneal  reflec- 
tions as  obtained  with  another 
type  of  modern  ophthalmom- 
eter. 


7.  The  patient's  head  should  be  in  such  a  position  that  the 
eyes  are  horizontal,  because  if  the  head  were  tilted  and  one  eye 
were  higher  than  the  other,  the  apparent  location  of  the  prin- 
cipal meridians  would  not  l)e  true,  but  might  be  off  as  much 
as  ten  to  fifteen  degrees  from  their  correct  positions.  This 
requirement  is  readil}-  fulfilled  as  follows:  .After  having 
focused  upon  one  eye  and  so  arranged  affairs  that  the  two  inner 
reflections  from  the  right  eye,  for  example,  are  practically  cen- 
tered with  respect  to  the  intersection  of  the  cross-hairs,  then 
immediately  swing  across  to  the  left  eye  and  if  the  heatl  is  in 
good  adjustment  from  the  standpoint  just  mentioned  a  similar 
condition  will  e.xist  with  respect  to  the  location  of  the  two 
inner  images  rellected  from  this  eye  with  respect  to  the  cross- 
hairs. In  this  manii)ulation  the  eye-co\  er  should  l)e  swung  to 
the  median  position  so  that  both  eyes  may  be  kept  uncwvered. 

8.  The  corneal  reflections  of  the  two  mires  are  doubled  by 
the  bi-refringent  i)ristns.  and  of  the  four  images  seen  in  the 
field  the  two  inner  ones  only  (which  should  ordinarily,  with 
the  dial  jicjinter  set  at  about  the  A5  diopter  point,  be  cU)se 
l(jgeilier  or  possibly  o\erla|))  being  regarded.  These  images 
are  then  made  to  fall  uptJii  the  point  of  intersectit»ti  of  the 
cross-hairs  in  the  eye-]»iece  b\  a  inaiiipulatitm  of  the  telescope 
through  a  mo\emenl  (if  the  same  up  or  down  and  by  swinging 


OPHTHALMOMETRY 


285 


it  upon  its  vertical  axis.  When  this  adjustment  is  obtained 
the  telescope  should  be  clamped  so  that  it  will  not  move  from 
its  position  while  various  observations  are  being  made. 

9.  The  mires,  (or  prisms  in  the  case  of  stationary  mires), 
are  moved  along  the  arc  by  means  of  the  milled  head  or  worm- 
device  on  the  back  of  the  large  dial,  until  the  two  middle 
images  are  just  in  contact  at  their  edges.  The  telescope  is 
then  rotated,  by  grasping  each  end  of  the  arc  carrying  the  mires 
and  turning  with  a  circular  motion,  until  the  bisecting  lines 
through  each  of  the  mires  form  a  continuous  line  and  the  posi- 
tions of  the  mires  on  the  arc  are  readjusted,  if  necessary,  for 
contact.  This  locates  for  us  one  principal  meridian.  (It  is 
absolutely  useless  and  valueless  to  employ  the  terms  primary 
and  secondary  meridians  in  the  sense  in  which  they  are  often 
used  ;  all  the  two  words  connotate  is  first  and  second  positions 
of  alignment). 


r — 

_ 


ABC 

Figure  5.  Corneal  reflections  of  the  mires.  (A)  Images 
overlapping  and  off  a  principal  meridian.  (B)  Images 
overlapping  but  in  a  principal  meridian.  (C)  Images  in 
alignment  and  in   contact  in  the  horizontal  meridian. 

10.  If  the  cornea  is  spherical,  the  central  images  will  remain 
in  contact  as  the  arc,  which  carries  the  mires,  is  rotated  about 
the  horizontal  line  or  axis  of  the  telescope.  If  there  is  corneal 
astigmatism,  the  distance  between  the  images  will  vary  as  the 
arc  is  rotated  into  various  meridians  on  account  of  the  varia- 
tion of  curvature  or  the  toric  form  of  an  astigmatic  surface. 
Furthermore,  the  images  will  have  a. curious  excentric  sliding 
motion  with  respect  to  each  other.  The  two  principal  meri- 
dians should,  under  normal  conditions,  be  90  degrees  apart. 
To   locate  a   principal   meridian,   therefore,   we   have    only   to 


286  OPHTHALMOMETRY 

locate  that  direction  in  which  the  black  central  lines,  or  their 
equivalents,  on  the  mires  are  continuous  with  each  other  as 
viewed  through  the  telescope,  since  this  occurs  only  when  the 
meridian  parallel  to  the  plane  of  the  mires  is  spherical. 

11.  Various  instructions  are  given  with  different  makes  of 
instruments  as  to  the  method  of  setting  the  pointers  attached 
to  the  dial  or  the  manipulations  of  the  drum  or  dial  arrange- 
ments for  the  adjustment  of  the  prisms  necessary  for  contact 
of  the  mires  in  different  meridians  and,  through  them,  the 
determinations  of  the  corneal  astigmatic  error  if  such  exists. 
The  writer  recommends,  for  one  or  two  reasons  which  will  be 
considered  directly,  that  the  power  in  each  principal  meridian 
and  the  meridian  measured  be  recorded  for  each  eye.  Thus, 
if  the  mires  are  in  the  horizontal  (0-180  degree)  meridian,  we 
measure  the  power  in  that  meridian.  This  is  mathematically 
and  physically  equal  to  the  power  of  a  cylinder  of  like  power 
to  that  determined  by  the  ophthalmometer  with  the  axis  of  the 
cylinder  representing  this  power  at  right  angles  to  the  meri- 
dian measured.  For  instance,  if  the  mires  are  in  contact  in 
the  90th  meridian,  (i.  e.,  with  the  central  lines  continuous  in 
the  vertical  direction)  we  are  measuring  the  corneal  power  in 
the  vertical  meridian ;  let  us  assume  this  to  be  45  diopters. 
We  should,  therefore,  make  our  record  as  showing  the  equiva- 
lent corneal  power  of  the  eye  in  the  90th  meridian  as  45  D. 
cyl.  ax.  180.  In  practice,  therefore,  we  simply  record  both  the 
power  and  the  meridian  measured  somewhat  as  illustrated  in 
the  following: 

Ophthalmomctric  Data. 

Meridian 

Eye                                Power  Measured 

Right                             45  D.  *X) 

48  D.  180 

Left                              43  D.  100 

42  D.  10 

These  data  may  be  conveniently  recorded,  as  shown  in  the 
accompanying  diagram,  by  the  use  of  a  circle  and  a  cross 
drawn  in  the  position  of  the  jirincipal  nuTi(li;ins,  i;uh  iiuMi- 
dian  measured  being  niaikcd  with  the  \;iluc  of  tin-  power 
possessed. 


OPHTHALMOMETRY 


287 


O.D. 


O.S. 


Figure  6.  Diagram  illustrative  of  a 
simple  method  of  recording  the  prin- 
cipal meridians  and  their  powers  in  a 
pair  of  eyes. 


In  the  case  of  the  right  eye  we  have  recorded  an  assumed 
corneal  astigmatism  of  3  diopters.  There  is  no  direct  evidence, 
per  se,  that  a  minus  or  phis  cylindrical  correction  will  be  re- 
quired in  the  prescription,  since  that  phase  of  the  data  must 
rest  upon  whether  or  not  the  eye  has  simple  hyperopic  astig- 
matism, simple  myopic  astigmatism  or  compound  or  mixed 
astigmatism.  Such  data  must  come  from  retinoscopic  and  sub- 
jective tests.  Neither,  again,  have  we  any  reason  to  believe 
that  the  ophthalmometrically  determined  astigmatism  will 
agree  in  amount  or  exact  axis  with  that  finally  fixed  upon  and 
judged  to  be  the  correct  astigmatic  finding.  We  shall  discuss 
this  matter  thoroughly  in  other  paragraphs.  However,  we  do 
have  a  condition  of  corneal  astigmatism  against  the  rule,  as  it 
is  known,  since  the  vertical  meridian  has  less  power  than  the 
horizontal  meridian.  The  difiference  in  corneal  curvatures  in 
the  illustration  chosen  is  represented  by  the  equivalent  of  a 
3  D.  cyl.  ax.  180  placed  in  contact  with  the  cornea.  This 
would  add  three  diopters  of  power  to  the  vertical  meridian 
and  hence  make  both  powers  equal  to  48  diopters.  Or  again, 
a  — 3  D.  cyl.  ax.  90  might  be  thought  of  as  being  in  contact 
with  the  cornea ;  this  would  take  away  three  diopters  of  power 
from  the  horizontal  meridian  and  hence  make  both  powers 
equal  to  45  diopters.  This  would  mean,  in  other  words,  that 
we  would  have  made  through  the  addition  of  the  cylinder  a 
spherical  refracting  surface  of  the  cornea.  If  there  were  no 
other  sources  of  astigmatic  error  in  this  eye,  which  would 
either  add  to  or  subtract  from  the  total  corneal  astigmatism, 
and  if  account  need  not  be  taken  of  the  efifectivities  of  lenses 


288  OPHTHALMOMETRY 

at  appreciable  distances  from  the  eye.  we  should  expect  to 
find  by  retinoscopic  methods  an  amount  of  astigmatism  approx- 
imating that  just  specified. 

In  the  case  of  the  data  given  with  reference  to  the  left  eye. 
however,  we  have  a  corneal  astigmatism  with  the  rule,  in 
which  the  vertical  meridianal  power  is  greater  than  the  hori- 
zontal, thereby  demanding  in  the  illustration  chosen  a  one 
diopter  cylinder  to  make  both  meridians  of  equal  power.  This 
may  be  accomplished,  (from  the  standpoint  of  the  ophthalmo- 
metric  findings  but  not,  it  is  to  be  remembered,  necessarily  in 
exact  agreement  with  the  amount  of  astigmatism  determined 
by  other  methods)  by  either  a  1  D.  cyl.  ax.  100.  which  would 
add  power  to  the  horizontal  meridian  and  therefore  make  both 
meridians  of  43  D.  power  or  by  the  use  of  a  — 1  D.  cyl.  ax.  10, 
which  would  take  away  power  from  the  vertical  meridian  and 
cause  both  meridians  to  ha\e  equal  powers  of  42  D. 

We  shall  have  occasion  to  discuss  some  very  vital  points 
with  respect  to  the  significance  of  these  corneal  findings  in 
paragraphs  devoted  to  a  discussion  of  theoretical  and  practical 
interpretations  of  ophthalmomctric  findings. 

In  closing  these  remarks  on  the  manipulation  and  use  of 
the  ophthalmometer  we  desire  to  say  that  we  have  referred  to 
the  classic  form  of  instrument  with  movable  mires.  .Xnoiher 
excellent  make  of  instrument  has  stationary  mires  with  the 
adjustment  of  images  made  through  an  arrangement  controll- 
ing the  prisms.  In  the  statements  we  have  made  it  is.  there- 
fore, necessarv  to  speak  only  of  the  fact  that  adjustment  of 
images  by  the  prism  mo\ement  de\  ice  is  synonyniou>  with 
nKjvement^  of  the  mires  along  the  arc. 

SOME  SIMPLE  THEORETICAL  CONSIDERATIONS, 
h'or  all  practical  puri)oses  the  anterior  surface  of  the  normal 
ccjrnea  may  be  considered  as  a  convex  mirror.  We  know  that 
the  sizes  of  the  images  reflected  in  plane  mirrors  are  of  the 
same  dinnMisions  as  those  of  the  objects.  With  conxex  mir- 
rors the  image  is  always  apparently  behind  the  mirror  and 
nuich  smaller  than  the  object.  The  smaller  the  radius  of  cur- 
vature or  the  greater  the  curvature  of  the  convex  surface  the 


OPHTHALMOMETRY 


289 


smaller  will  be  the  size  of  the  image.  Hence,  if  the  convex 
mirror  should  be  more  curved  in  one  meridian  than  in  another, 
the  reflection  of  a  circular  object  would  no  longer  be  round 
but  would  be  oval,  with  the  longer  axis  parallel  to  the  meridian 
of  least  curvature. 


Figure  7.  Illustrating  the  fundamental  physical 
priciples  of  reflection  from  convex  mirrors  or  from 
the  cornea. 

The  accompanying  diagram  shows  the  fundamental  prin- 
ciples of  the  reflection  of  images  by  convex  mirrors  or  the 
convex  cornea.  M  M  represents  the  cornea  and  A  B  is  to  be 
considered  as  a  single  mire.  The  distance  M  A  is  large  (say 
a  foot)  as  compared  with  the  radius  of  curvature  of  the  cornea 
which  is,  on  the  average,  about  7.8  millimeters.  The  image  of 
A  B,  i.  e.  A'  B',  will  be  formed  back  of  the  mirror  and  prac- 
tically at  its  focal  point,  F  =  r/2. 

The  simple  ophthalmometric  formula  is 

2f  I 


r  = 


O 


in  which  r  is  the  radius  of  curvature,  f  is  the  object  distance, 
O  the  size  of  the  object  and  I  the  size  of  the  image. 

This  is  the  equation  which  must  be  satisfied  in  the  construc- 
tion of  the  ophthalmometer  and  from  which  it  may  be  cal- 
culated (see  Sheard's  Physiological  Optics,  pages  51-53)  that 
the  strength  of  the  doubling  prism  used  in  modern  ophthal- 
mometers is  so  arranged  as  to  give  a  displacement  of  exactly 
3  millimeters. 

The  ophthalmometer  is  used  to  find  the  anterior  focal  power 
of  the  cornea.     The  dial  or  disc  is   usuallv  marked  in   both 


290  OPHTHALMOMETRY 

diopters  and  in  millimeters  radius  of  curvature.  So  many 
errors  have  been  seen  by  the  writer  in  printed  statements 
having  to  do  with  the  calculations  of  these  dioptral  powers  and 
radii  that  he  may  be  pardoned  a  brief  reference  to  the  correct 
equation  and  its  use.  The  anterior  corneal  focal  power  is  rep- 
resented by 

1000  (n  — I) 

D,= 

r 
where  r  is  the  radius  of  curvature  in  millimeters  and  .n  the 
index  of  the  cornea.  It  is  quite  commonly  stated  that  the 
index  of  the  cornea  is  the  same  as  that  of  the  aqueous  or 
water,  namely  1.333.  Such  is  not  true,  for  the  cornea  has  an 
index  commonly  assigned  as  1.3375.  If  this  number  is  used  in 
the  above  relation  we  find  that  a  radius  of  7.S  mms.  corresponds 
to  a  power  of  45D.  To  anyone  who  will  take  the  trouble  to 
check  out  the  accuracy  of  the  markings  in  diopters  power  and 
in  millimeters  radius  of  curvature  by  the  foregoing  equation 
the  following  may  be  of  service : 


Power  in  diopters.... 
Radius  in  millimeters. 


40     41      42     43      44     45      46     47     48     4*^ 
8.44  8.23  8.04  7.85  7.67  7.50  7.34  7.18  7.03  6.89 


POSSIBLE  VALUE  OF  OPHTHALMOMETRIC  CURVATURES  IN 
HYPEROPIA  AND   MYOPIA. 

It  is  probable  that  the  ophthalmometric  mcasurcnunls  of 
curvature  do  lend  information  of  value  in  many  cases  of  hyper- 
opia and  myopia.  Calculations  show  that  an  increase  or 
decrease  of  one  millimeter  in  length  of  an  (.-ycball  along  the 
a.xis  corresponds  to  twtj  and  oiu'-lialt  to  lliric  diopters  of 
refractive  change.  It  is  probable  that  variations  in  the  Ungtlis 
of  eyes  through  the  ocular  axes  are  greater  than  are  the  \aria- 
tions  in  corneal  curvatures.  But  variations  in  the  cur\  ature  of 
the  cornea  affect  the  refraction  to  a  greater  extent  than  do 
variations  in  length.  Vox  a  variation  of  t)ne  millimeter  in  the 
anterior  c:jrneal  radius  of  curvatiu'e  corresi)onds  to  about  six 
diopters.  lience,  hiKh  or  low  corneal  curvatures  may  be 
important  etiological   factors   in   miopia   and   hypi-ropia   or   in 


OPHTHALMOMETRY  291 

tendencies  thereto.  If,  then,  the  eye  is,  for  example,  highly 
hyperopia  (let  us  say  6D.)  and  the  corneal  measurements 
should  indicate  curvatures  as  low  as  40  to  41  diopters  we 
should  be  justified,  we  believe,  in  including  that  the  hyperopia 
was  of  a  curvature  type  rather  than  being  axial,  or  possibly 
lenticular  in  nature.  The  possibilities  for  vision,  when  cor- 
rected, might  be  expected,  other  features  not  proving  the  same 
to  be  impossible,  to  be  raised  to  something  approximating  the 
normal  standard.  But  if  an  amblyopic  eye,  of  6  diopters  for 
example,  were  under  examination  and  the  curvatures  of  the 
cornea  were  about  normal  and  of  practically  the  same  values 
as  those  of  the  other  eye,  we  should  be  compelled  to  look  else- 
where for  the  seat  of  the  hyperopia  and  of  the  amblyopia.  The 
values  of  the  visual  acuity,  before  and  after  correction,  would 
then  depend  largely  upon  the  history  of  the  case  and  the  pre- 
vious treatment. 

In  myopia  there  are  two  important  points  which  should  be 
settled :  the  measurement  of  the  radius  of  curvature  of  the 
cornea  and  the  examination  of  the  fundus.  Let  us  say  that 
the  average  curvature  of  the  cornea  is  7.65  mm.  as  a  very  fair 
estimate.  If  the  radius  of  curvature  is  less  than  this,  it  indi- 
cates a  cornea  with  a  sharper  curvature  and  hence  greater 
refractive  power;  whereas,  if  the  radius  be  longer  the  cornea 
is  flatter  and  less  powerful  refractively.  In  general,  then,  a 
short  radius  of  curvature  in  myopia  indicates  that  the  case  is 
of  a  benign  refractive  character  in  which  the  rays  are  brought 
to  a  focus  in  front  of  the  retina  by  virtue  of  the  excessive 
refractive  power  of  the  cornea.  If,  however,  a  reasonable 
amount  of  myopia  exists  (i.  e.  two  diopters  or  more)  and  the 
radius  of  the  cornea  is  large  or  even  normal,  there  is  then  a 
fair  indication  that  the  myopia  present  must  be  due  to  the  dis- 
placement of  the  retinal  tissues  backward.  This  would  indi- 
cate the  progressive  axial  type  of  myopia.  The  examination  of 
the  fundus  by  means  of  the  ophthalmoscope  is  of  considerable 
value  here.  For,  if  there  are  no  changes  present  in  the  fundus 
of  a  person  with  a  short  corneal  radius  it  is  perfectly  safe  to 
say  that  the  case  is  a  non-progressive  one.  The  absence  of 
fundus  changes  is  not  so  conclusive,  however,  in  flat  corneas, 


292 


OPHTHALMOMETRY 


since  the  myopia  is  evidently  axial  and  not  refractive  and 
changes  are  likely  to  develop  later.  If,  perchance,  those 
changes  are  present  which  indicate  an  inflammatory  softening 
of  the  structures  at  the  posterior  pole,  then  the  diagnosis  of 
progressive  myopia  can  be  made  with  considerable  certainty. 

IRREGULAR  ASTIGMATISM. 
Before  the  days  of  the  modern  ophthalmometer  and  other 
methods  of  diagnosis  no  class  of  refractive  cases  was  more 
troublesome  or  difficult  to  handle  than  those  with  irregular 
curvature  of  the  cornea.  In  the  ophthalmometer,  the  images 
of  the  mires  are  of  value  in  detecting  irregular  astigmatism, 


Normal  Cur-vaiures 


FlKure  10.  rppcr  illaKniiii  shows  th.-  iimnial  re- 
flections of  mlrcK  In  ii  principal  ni<>rlillan.  The 
Jf)wcr  rllaKram  H!)f)WH  the  cllalorllun  In  a  oase  of 
irrfgujar   aBllgniallsin. 


OPHTHALMOMETRY  293 

for  they  show  distortion  of  the  corneal  image  and  the  irregu- 
larity of  the  outlines  of  the  mires  when  corneal  irregularities 
exist.  Such  irregular  astigmatism  is  also  shown  when  there  is 
no  position  of  the  mires  in  which  the  corneal  reflections  can 
be  made  to  form  one  continuous  line.  This  condition  of 
affairs  is  roughly  diagrammed  in  the  figure  given  below,  in 
which  the  upper  illustration  shows  the  normal  condition  of 
afifairs  while  the  lower  diagram  attempts  to  represent  the  dis- 
tortion with  lack  of  continuity  of  the  central  lines  of  the  mires 
in  irregular  astigmatism.  The  best  procedure,  then,  is  to  find 
the  position  of  nearest  approach  to  normal  corneal  reflections 
in  two  meridians  as  nearly  at  right  angles  to  each  other  as 
possible.  Or,  to  state  it  another  way,  regular  astigmatism  may 
be  present  in  addition  to  the  irregular  form :  there  are  then, 
in  general,  two  positions  at  which  these  lines  become  more 
nearly  continuous  and  straight  and  afford  more  nearly  normal 
contact  than  elsewhere,  thus  indicating  the  meridians  of  the 
regular  astigmatism. 

The  ophthalmometric  examination  is  so  delicate  in  the  dis- 
tortion test  through  the  images  of  the  mires  that  the  smallest 
irregularities  of  the  corneal  surface  may  be  detected.     Follow- 


TOZ 

Figure  11.  The  upper  row  of  letters  is 
a  reproduction  of  a  well-known  set  of 
letters  on  a  test-card;  the  lower  set 
represents  these  letters  as  diagrammed 
for  us  by  the  patient  in  a  case  of 
irregular    astigmatism. 


ing  such  an  examination,  there  should  be  made  a  close  inspec- 
tion of  the  surface  of  the  cornea  by  oblique  illumination  and 
an  examination  for  minute  opacities,  facets  and  pits.  Likewise, 
the  ophthalmoscopic  examination  may  afford  evidence  of  the 


294  OPHTHALMOMETRY 

presence  of  such  irregularities,  for  the  writer  has  had  cases  in 
which,  when  viewed  at  the  proper  angle  such  as  to  include  the 
irregularity,  a  doubleness  of  optic  disc,  etc.  was  visible.  Sub- 
jectively the  condition  may  often  be  detected  by  the  distortion 
and  grotesque  forms  which  the  larger  letters  in  our  test  charts 
assume.  One  or  two  patients  have  sketched  these  for  us  and 
we  are  appending  one  set  taken  from  our  history  cards.  The 
first  row  of  letters  represents  normal  conditions ;  the  second 
set  as  seen  by  this  patient. 

In  passing,  let  it  be  said  that  we  have  found  that  the  use  of 
the  stenopaic  slit  in  these  cases  of  irregular  astigmatism,  with 
the  determination  of  the  meridians  of  best  and  poorest  vision 
and  their  independent  correction,  to  be  ultimately  combined 
into  a  sphere-cylinder  combination,  has  been  of  great  service. 
And  the  ophthalmometer  aided  greatly  in  the  location  of 
these  two  meridians. 

To  illustrate  the  points  we  have  been  discussing  we  are  cit- 
ing two  cases.  It  should  be  remarked  that  many,  if  not  most, 
of  these  conditions  of  irregular  astigmatism  are  the  sequelae 
of  various  types  of  ulcers. 

KERATOCONUS. 
Keratoconus  is  a  non-inflammatory  conical  protrusion  of  the 
center  of  the  cornea  and  is  due  to  a  thinning  of  this  membrane 
whereby  it  is  unal)le  to  resist  the  normal  intra-ocular  pressure. 
Conical  cornea  causes  myopia  and  astigmatism  and  seriously 
interferes  with  vision  even  after  the  best  i)ossible  corrections 
with  lenses.  The  most  valuable  objective  test  is  that  with  the 
ophthalmometer.  The  retinosco|)ic  procedures  may  I)e  of 
value.  The  subjective  tests  with  the  trial  lenses  are  often 
facilitated  by  the  employment  of  the  stenopaic  slit.  In  kera- 
toconus there  is  always  more  or  less  irregular  astigmatism. 
The  oplitliahnonielcr  is  not  generally  capable  of  locating  the 
axes  with  exactness  or  of  measuring  their  jiowers.  but  it  does 
afford  a  method  of  close  api)roxiination.  The  ophthaliuometer 
and  the  Placido's  disc  enable  one  to  select  the  area  in  or  near 
the  visual  area  which  is  best  suited  for  \  isual  purposes.  In 
the    high    amounts   of   astigmatism    that    commonly    occur    in 


OPHTHALMOMETRY 


295 


Figure  12.  Keratoscopic  figures  of  a  cornea  presenting 
a  considerable  astigmatism  at  tlie  central  part.  (After 
Javal.) 


Figure  13.     Images  from  the   Placido's  disc   in   a  case  of 
keratoconus.      (After  Javal.) 


296  OPHTHALMOMETRY 

conical  cornea  it  is  necessary  to  add  an  extra  prism  to  the 
ophthalmometer.  This  decreases  the  sizes  of  the  images  of 
the  mires  one-half,  but  enables  the  operator  to  measure  much 
higher  degrees  of  astigmatism. 

While  the  power  of  the  normal  cornea  varies  between  40  and 
48  diopters,  higher  values  are  found  in  cases  of  conical  cornea. 
Instances  of  60  to  80  diopters  are  occasionally  found :  in  fact, 
Cordiale  found  a  case  in  which  the  curvature  was  about  100 
diopters,  corresponding  to  a  radius  of  3.4  mms.  \'arious  oph- 
thalmometric  examinations  in  such  cases,  made  by  Cordiale, 
show  that  the  dioptric  powers  of  the  cornea,  at  points  one  or 
two  millimeters  to  the  nasal  or  temi)oral  side  of  the  visual 
line,  change  as  much  as  20  or  more  diuj^ters,  whereas  in  the 
normal  cornea  there  is  practically  a  uniformity  of  curvatures 
over  this  same  region.. 

ADVANTAGES  OF  OPHTHALMOMETHIC  MEASUREMENTS. 
As  mentioned  in  other  sections  of  this  \olume,  the  ophthal- 
mometer is  designed  to  measure  the  radius  of  curvature  of  the 
cornea  and  to  thereby  determine  its  refracting  power.  The 
facts  which  are  obtainable  through  the  making  of  these  meas- 
urements include: 

(\)  Whether  the  corneal  curwitures  are  the  same  in  all 
meridians  or  not:  if  there  is  no  difference,  then  there  is  no 
corneal  astigmatism. 

(2)  Whether  the  radius  of  curxature  is  shorter  or  longer 
than  the  average.  This  is  of  \alue  e\en  though  no  corneal 
astigmatism  is  found,  for  the  reason  that  a  dilTerence  of  one 
millimeter  in  the  radius  of  curvature  of  a  cornea  corresponds 
jiractically  U)  six  diopters  of  refracting  pitwer.  It  is  com- 
UKjnly  found  that  an  eye  having  a  cornea  with  a  railius  of 
one-half  nnlhnieter  or  more  shorter  than  the  a\  eraj^e  is  myttpic  : 
in  turn,  one  witli  a  radius  of  cur\ature  one-half  niiniineter  or 
more  longer  than  the  a\erage  is  hy|»eropie.  Ilence.  the  oph- 
thalmometric  measurements  may  be  of  sei\  ice  in  indicating 
refracli\e,  cur\alure  or  axial  h\  pt.'!  iii)i;i  of  myopia. 

(3)  Whether  the  corni'a  has  a  toroidal  cutm-;  if  so,  corneal 
astigmatism    is    pii-seut.      'I  he    principal    meridians    and    their 


I 


OPHTHALMOMETRY  297 

powers  may  be  located  accurately.  The  operator  will  not  be 
able  to  determine  whether  a  plus  or  minus  cylmder  should  be 
accepted  by  the  patient,  but  he  will  be  able  to  determine  where 
to  place  the  axis  of  the  plus  or  minus  cylinder  indicated,  should 
either  be  eventually  accepted,  in  so  far  as  corneal  conditions 
are  concerned.  There  are  possible  exceptions  to  this  last  state- 
ment, especially  in  cases  of  cyclophoria  at  close  points  when 
there  is  an  absence  of  cyclophoria  at  distance.  Of  course,  the 
reader  is  not  to  imply  from  the  foregoing  statements  that  the 
ophthalmometric  findings  as  to  axis  are  infallible :  they  simply 
show  corneal  conditions. 

(4)  We  may  determine  whether  irregular  astigmatism  or 
keratoconus  is  present  or  not  and  to  what  extent.  Any  asym- 
metry of  this  character  will  be  readily  detected,  for  the  images 
of  the  mires  will  be  distorted.  Frequently  the  images  of  the 
mires  lose  all  resemblance  to  the  mires  themselves. 

(5)  We  may  determine  whether  the  corneas  of  the  two  eyes 
are  similar  or  dissimilar  and  whether  the  corneal  astigmatism, 
if  present,  is  symmetrical  or  unsymmetrical.  To  illustrate :  if 
the  meridian  of  greatest  refraction  is  at  90  degrees  (or  180 
degrees)  in  both  eyes,  the  patient  then  scqs  vertical  or  hori- 
zontal lines  sharply  (dependent  upon  the  error)  without  tort- 
ing  the  eyes.  If  the  meridians  are  90  degrees  and  70  degrees 
respectively,  the  patient  will  be  unable  to  see  the  same  lines 
distinctly  with  both  eyes  unless  he  tilts  his  head  some  10 
degrees  in  order  to  bring  the  two  principal  meridians  into  some 
thing  like  symmetrical  positions,  or  else  the  oblique  muscles 
will  have  to  tort  the  eyes  in  the  interest  of  clear  binocular 
single  vision.  It  is  likewise  true  that  the  condition  of  slightly 
blurred  yet  equal  or  similar  images  upon  the  maculae  of  both 
eyes  is  less  destructive  to  good  vision  than  the  condition  of  a 
blurred  image  upon  the  macula  of  one  eye  and  a  sharp  image 
upon  the  macula  of  the  other.  Great  care  is  necessary  to  see 
that  the  cylinders  are  correctly  placed  in  order  that  no  tilting 
of  the  head  or  torting  of  the  eyes  (cyclophoria)  shall  be 
induced  for  the  purpose  of  obtaining  similar  images  upon  the 
two   maculae.     Hence,   we   particular!})    desire   that    the   eye 


298  OPHTHALMOMETRY 

remain  in  its  position  of  rest.  If  the  axes  of  the  cylinders  are 
incorrectly  placed,  asymmetrical  meridians  may  be  produced, 
causing  the  eyes  of  the  patient  to  be  torted  or  the  head  to  be 
tilted,  thereby  producing  more  or  less  discomfort. 

It  is  often  of  the  greatest  value  to  note  the  expressions  of 
opinion  of  scientific  men  upon  various  phases  of  the  work  in 
which  they  are  interested.  Opinions  from  as  widely  separa- 
ted geographical  sources  are  equally  to  be  desired :  we  may, 
therefore,  be  pardoned  the  insertion  of  a  lengthy  quotation 
from  an  address  on  "Errors  of  Refraction"  (British  Journal  of 
Ophthalmology,  Vol.  II.  June,  1918)  by  Lieut.-Colonel  R.  H. 
Elliot  of  London,  England.  He  says:  "The  uses  of  an 
astigmometer,  as  they  appear  to  me,  are  as  follows:  -(1)  The 
instrument  will  often  give  the  exact  axis  of  the  astigmatism. 
(2)  When  the  axes  of  greatest  and  least  astigmatism  are  not 
at  right  angles  with  each  other,  it  will  point  out  the  discrep- 
ancy and  give  both  readings.  (3)  Though  it  only  measures 
the  corneal  astigmatism,  and  though  a  correction  has  almost 
invariably  to  be  made  to  its  readings,  it  furnishes  valuable 
data  in  a  large  number  of  cases.  (4)  In  asthenopic  patients, 
with  low  astigmatism  against  the  rule,  it  will  point  out  the 
defect  unerringly,  and  indicate  the  need  for  a  cycloplegic 
retinoscopy,  even  though  the  vision  is  fully  normal.  (5)  In 
cases  of  high  hyperopic  and  myopic  astigmatism,  which  present 
great  difficulties  for  satisfactory  retinoscopy,  its  indications 
prove  more  than  valuable  in  a  large  number  of  instances.  (6) 
In  cases  with  deep  medial  opacities,  sufficiently  dense  to  make 
retinoscopy  unsatisfactory,  the  correction  of  the  corneal  astig- 
matism, made  by  the  aid  of  the  astigmometer,  may  prove  of 
much  use.  (7)  In  the  routine  examination  of  presbyopcs,  be- 
fore prescribing  a  reading  correction,  astigmometry  shortens 
the  examination  and  adds  precision  to  it.  (8)  In  children, 
restless  patients,  and  imbeciles,  it  sometimes  proves  a  valuable 
auxiliary  to,  or  even  substitute  for,  retinoscopy.  (9)  As  a 
means  of  ascertaining  wlu-thcr  any  surgical  procedure  or 
corneal  affection  alters  the  corneal  astigmatism,  it  is  un- 
rivalled.     Tncidcntallv,  its   use   in   this  connection   sometimes 


OPHTHALMOMETRY  299 

furnishes  information  of  great  interest.  May  I  cite  one  ex- 
ample? The  corneal  astigmatism,  found  after  cataract  extrac- 
tion as  shown  by  the  astigmometer,  is  markedly  in  excess  of 
the  total  astigmatism  present.  There  is,  therefore,  a  com- 
pensatory factor  in  action  and  this,  I  think,  is  to  be  sought  at 
the  anterior  surface  of  the  vitreous.  I  am  not  aware  that  this 
observation  has  ever  been  put  on  record,  but  any  of  you  can 
confirm  it  for  yourselves.  (10)  It  will  unerringly  and  imme- 
diately show  the  presence  of  slight  nebulae  of  the  cornea,  irreg- 
ular corneal  astigmatism,  disparities  between  the  curvature  of 
one  and  another  part  of  the  cornea,  such  as  produce  scissor 
movements  in  retinoscopy,  and  the  existence  of  conical  cornea." 
Furthermore,  there  are  the  reports  of  John  Rowan  (British 
Medical  Journal,  1912)  in  which  the  astigmatism  in  a  thousand 
eyes  was  measured,  first  by  the  ophthalmometer  of  Javal  and 
Schiotz  and  then  by  retinoscopy,  with  atropin  or  homotropin 
as  a  cycloplegic.  The  results  of  his  measurements  show  that, 
out  of  the  one  thousand  eyes  examined,  the  total  astigmatism 
and  the  corneal  astigmatism  were  in  agreement  in  nearly  fifty 
per  cent  of  the  cases.  This  is  surely  an  interesting  and  valu- 
able conclusion  and  comes  as  a  brisk  rejoinder  to  those  who 
minimize  the  value  of  the  ophthalmometer  in  ocular  refrac- 
tion to  the  point  of  stating  that  it  is  of  no  value  whatever. 

DISCUSSION    OF    THE    RELATIONS    BETWEEN    CORNEAL 
AND  TOTAL  ASTIGMATISM. 

Quite  a  difference  is  often  found  between  the  ophthalmo- 
metric  and  subjective  measurements.  This  was  first  pointed 
out  by  Bonders  and  later  by  Knapp  who  attributed — with  a 
fair  degree  of  accuracy  in  many  cases — this  difference  to  an 
astigmatism  of  the  crystalline  lens  which  would  act  in  a  con- 
trary direction  to  that  of  the  corneal  astigmatism.  Javal, 
Pfliiger,  Tscherning  and  others  have  investigated  the  relations 
between  corneal  and  total  subjective  astigmatism. 

The  rules  of  Javal,  commonly  accepted  by  most  investi- 
gators and  refractionists  as  being  a  first  order  approximation 
to  the  truth  and  hence  of  much  value,  are : 


300  OPHTHALMOMETRY 

(1)  If  there  is  no  ophthalmometric  astigmatism,  we  gen- 
erally find  a  slight  subjective  astigmatism  against  the  rule. 
This  condition  would,  of  course,  call  for  corrections  involving 
plus  cylinders  at  or  in  the  vicinity  of  180  degrees  or  minus 
cylinders  at  or  in  the  vicinity  of  90  degrees. 

(2)  If  the  ophthalmometric  astigmatism  is  against  the  rule, 
the  subjective  astigmatism  is  usually  against  the  rule  and  of 
a  greater  amount. 

(3)  If  the  ophthalmometric  astigmatism  is  with  the  rule  and 
of  a  value  intermediate  between  one  and  three  diopters,  the 
subjective  astigmatism  generally  differs  only  slightly  from  it. 

(4)  If  the  ophthalmometer  gives  an  astigmatism  with  the 
rule  and  greater  than  three  diopters,  the  subjective  astigma- 
tism is  also  with  the  rule  and  frequently  greater. 

Javal  expressed  the  difference  between  the  subjective  astig- 
matism (Ass)  and  the  ophthalmometric  astigmatism  (Asc)  by 
the  empirical  rule  or  formula  that 

Ass  =  k  -f  p.Asc 
in  which  k  and  p  are  two  constants,  k  being  approximately 
0.5D  against  the  rule  and  p  having  a  value  of  1.25,  hence  a 
multiplying  factor  a  little  greater  than  unity.  We  can  physi- 
cally and  physiologically  account  for  the  significance  and  value 
of  the  term  k  but  we  know  of  no  investigator  or  writer  who  has 
satisfactorily  explained  why  the  factor  p  should  be  greater 
than  unity.  Since  k  has  a  value  of  0.5D.  against  the  rule,  it  is 
necessary  to  add  it  to  the  ophthalmometric  finding  in  cases  of 
astigmatism  against  the  rule  and  t(.)  subtract  it  in  cases  of 
astigmatism  with  the  rule. 

The   formula  gives  the   following  numerical   values   wliich 

are    found    to    check    reasonably    wiMl    in    ordinary,    everyday 

practice : 

Ophthalmometric  Against    the        I 

Astigmatism                     Rule               1  With  the   l\ule 

Subjective  Astig-  2             1           0          1  2       3       4       .^       6 

matism  3         1.75         0.5  10.75  2     3.25    4.5    5.75    7 

Certain  factors  must  be  considerctl  when  discussing  tiie 
reasons  why  ophthalmometric  astigmatism  ami  the  total  astig- 


OPHTHALMOMETRY  301 

matism  as  found  by  other  methods  are  often  not  in  conformity 
with  each  other:  (1)  the  positions  of  the  lenses  in  front  of 
the  eyes  and  the  influence  of  the  distance  of  the  correcting 
lens  from  the  eye ;  (2)  deformity  of  internal  surfaces  or  astig- 
matism due  to  the  forms  of  the  internal  ocular  surfaces;  (3) 
normal  or  abnormal  lenticular  astigmatism ;  (4)  astigmatic 
accommodation;  (5)  astigmatism  by  incidence  and  crystalline 
lens  obliquity ;  (6)  spherical  aberration  and  the  astigmatism 
in  the  different  zones  of  the  cornea  and  (7)  contraction  of  the 
recti  muscles. 

(1)  The  influence  of  the  distance  of  the  correcting  lens 
from  the  eye.  Years  ago  Javal  and  Pfliiger  carried  out  cor- 
rections of  astigmia  subjectively,  the  first  named  investigator 
using  plus  cylinders  whereas  the  second  used  minus  cylinders. 
The  subjects  were  examined  in  common,  and  the  ophthalmo- 
metric  data  afterward  compared  with  the  subjective  astigmatic 
findings.  It  was  found  that  Javal  obtained  higher  corrections 
than.  Pfliiger  throughout  the  whole  gamut  of  astigmatism 
tested.  The  astigmatism  of  the  eye  was,  under  their  methods 
of  testing,  constant :  hence  the  dift'erence  in  their  cylindrical 
corrections  for  various  ophthalmometric  determinations  is  due, 
in  part  at  least,  to  the  effectivity  of  plus  and  minus  correc- 
tions situated  at  a  certain  distance  from  the  eye.  We  must, 
therefore,  take  account  of  the  change  of  effectivity  of  cylinders 
as  they  are  placed  in  advance  of  the  cornea  and  as  they  are 
convex  or  concave :  hence,  apparent  changes  in  the  astigma- 
tism of  the  eye  may  arise  from  optical  sources.  To  illustrate : 
Suppose  there  is  found  with  the  ophthalmometer  4D.  of  actual 
corneal  astigmatism :  there  will  then  be  theoretically  required, 
if  the  eye  is  otherwise  emmetropic  and  the  cylinder  is  placed 
about  20  mm.  from  the  principal  plane  of  the  cornea,  either 
a  cylindrical  correction  of  -\-3.70D.  or  — 4.35D.,  dependent 
upon  the  nature  of  the  astigmatism.  (Vide:  American 
Encyclopedia  of  Ophthalmology,  Vol.  XIII,  pages  9814-18 
and  9873-76,  and  Sheard :  Physiological  Optics,  pages  94-98 
and  151-157.) 

(2)  Astigmatism  due  to  the  forms  of  the  internal  ocular 
surfaces.     About   seventy   per  cent  of  all   corneas    show   an 


302  OPHTHALMOMETRY 

astigmatism  of  from  0.5D.  to  ID.  at  the  anterior  corneal 
surface.  We  know  but  little  about  the  posterior  surface 
but  the  experiments  of  Stadfeldt  and  of  Tscherning  show 
that  the  vertical  meridian  of  the  posterior  surface  of  the 
cornea  possesses  a  more  pronounced  curvature  than  the  hori- 
zontal meridian,  whether  the  condition  in  toto  is  one  (jf 
astigmatism  with  or  against  the  rule.  Since  the  posterior 
surface  is  concave,  this  greater  curvature  in  the  vertical 
meridian  will  cause  an  astigmatism  against  the  rule.  It  may 
well  be  that  this  is  the  cause  of  the  small  subjective  astigma- 
tism against  the  rule,  generally  found  subjectively  when  the 
ophthalmometric  findings  show  no  anterior  differences  in 
various  meridians. 

Tscherning,  Stadfeldt  and  Awerbach  made  measurements 
upon  the  crystalline  lens,  using  an  instrument  known  as  the 
ophthalmophakometer.  The  anterior  surface  of  the  crystal- 
line in  all  cases  examined  showed  a  direct,  or  with  the  rule, 
astigmatism  while  the  posterior  surface  often  exhi])itetl  the 
inverse,  or  against  the  rule,  tyi)e.  We  are  to  judge  in  a  general 
way  from  the  results  of  their  experiments  that  the  crystalline 
surfaces  are  more  spherical  in  form  than  the  cornea. 

(3)  Lenticular  astigmatism.  Javal,  Nordenson,  Schiotz 
and  others  have  shown  that  the  lenticular  astigmatism  as  a 
rule  amounts  to  about  half  a  diopter.  This  may  be  called  the 
normal  lenticular  astigmatism,  just  as  we  have  about  the  same 
amount  of  astigmatism  normally  present  in  the  cornea.  In 
general  the  two  neutrali/.e  each  other  in  the  sense  that  they 
ofTset  each  other,  producing  the  e(iui\ak'nt  spherical  error. 

Dr.  Davis  (Essay  on  Refraction  and  Accommodation  of  the 
Eye:  The  American  Encyclopedia  of  Ophthalmology.  \ Ol. 
XIV,  page  11164)  writes:  "The  lenticular  astif^niatism.  as  a 
rule,  amounts  to  about  0.5UI).  to  0.731).,  as  pro\  cd  by  abiuuiaiit 
statistics,  and  it  has  been  slujwn  that,  in  actual  practice,  the 
corneal  astigmatism  is  diminished  ov  increased  that  amount, 
according  as  the  astigmatism  is  with  or  against  tlu-  rule.  The 
most  reasonable  explanation  to  be  given  for  the  necessity  itf 
deducting  0.51).  to  0.751).  from  the  reading  of  the  instrument 


OPHTHALMOMETRY  303 

when  the  astigmatism  is  with  the  rule  and  adding  a  like  amount 
when  the  astigmatism  is  against  the  rule  is  the  assumption 
that  in  corneal  astigmatism  with  the  rule  there  is  usually 
associated  a  lenticular  astigmatism  of  0.50  to  0.75D.  in  the 
same  meridian,  but  of  an  opposite  kind,  thereby  neutralizing 
that  amount  of  corneal  astigmatism.  In  astigmatism  against 
the  rule  there  is,  on  the  other  hand,  a  lenticular  astigmatism 
of  0.50  to  0.75D.  in  the  same  meridian  and  of  the  same  kind, 
thereby  adding  that  amount  to  the  corneal  astigmatism.  This 
can  be  explained  in  another  way.  In  corneal  astigmatism  with 
the  rule  the  lenticular  astigmatism  may  be  of  the  same  kind 
but  in  a  meridian  at  right  angles  to  the  corneal  astigmatism ; 
in  which  case,  if  the  meridian  at  error  in  both  the  cornea  and 
lens  were  myopic,  a  simple  myopia  of  0.50  to  0.75D,  would  be 
produced.  .  .  .  However,  the  first  explanation  is  more  likely 
to  be  the  true  one  and,  in  fact,  actual  measurements  (Bonders) 
have  shown  it  to  be  true ;  and  the  ophthalmophakometer  con- 
firms this  view."  ^ 

(4)  Astigmatic  accommodation.  Dolrowolsky  first  ex- 
pressed the  idea  that  astigmatic  persons  could  partly  correct 
their  defect  through  an  irregular  or  non-uniform  contraction 
of  the  ciliary  muscle,  thus  producing  a  deformity  of  the  crystal- 
line lens  in  the  opposite  direction.  Some  experimenters  and 
clinicians  admit  this  possibility  and  others  do  not.  There  are 
a  considerable  number  of  physical  facts  against  the  acceptance 
of  astigmatic  accommodation.  Still  these  and  other  facts  do 
not  bar  its  existence.  It  is  entirely  possible  from  anatomical 
standpoints,  since  it  is  possible  that  a  part  of  the  filaments 
which  go  to  the  ciliary  muscle  may  be  in  a  normal  condition, 
while  others  may  be  partially  paretic  or  insufficient  or,  again, 
over-active,  thus  producing  irregular  action  on  the  zonula. 
And  again,  subjectively  no  astigmatism  may  be  evidenced,  the 
vision  being  20/20  easily,  while  all  objective  tests  show  the 
presence  of  a  decided  astigmatism,  indicating  an  astigmatic 
accommodation.  Furthermore,  we  know  from  clinical  expe- 
rience that  the  correction  of  an  objective  astigmatic  condition 
lessens  discomfort  and  relieves  ocular  headaches  in  many  cases 
although  no  improvement  of  the  vision  may  result.     (Vide, 


304  OPHTHALMOMETRY 

Howe:  Muscles  of  the  Eye,  and  Sheard :  Physiological  Optics, 
page  144.) 

(5)  Astigmatism  by  incidence  and  lens  obliquity.  The 
pupil  of  an  eye  is  ordinarily  centered  with  respect  to  the  axis 
of  the  ocular  system.  The  point  of  fixation  is.  however,  upon 
the  visual  axis.  The  angle  between  the  optical  and  visual 
axes  is  generally  known  as  the  angle  alpha,  usually  of  the 
order  of  about  5  degrees.  Hence,  even  though  the  incident 
beam  along  the  axis  should  be  devoid  of  astigmatism,  the 
beam  along  the  visual  axis  will  not  be.  By  virtue  of  thisangle 
alpha  an  "against  the  rule"  astigmatism  arises  which  is,  on 
the  average,  about  one-half  to  three-c|uarters  of  a  diopter.  The 
following  table  is  self-explanatory  ui)on  this  point : 

Angle  Alpha   (Degrees)  

13  5  7  9 
Corneal  Astigmatism.. 0.02D.     0.13D.     0.351).     0.66D.     I.IID. 
Lenticular  Astig- 
matism  O.OID.     0.05D.     0.141).     0.26D.     0.44D. 

Total  Astigmatism 

(Diopters)    0.03D.     0.18D.     0.49D.     0.92D.     1.55D. 

Another  form  of  astigmatism  due  to  incidence  may  be  desig- 
nated as  astigmatism  due  to  lens  obliquity.  Vov  example,  if 
the  crystalline  lens  should  be  tilted  about  its  horizontal  axis, 
there  would  be  produced  thereby  an  astigmatism  against  the 
rule.  A  tilting  of  the  crystalline  lens  in  situ  of  10  degrees 
would  cause  an  astigmatism  of  about  0.5D.  The  Bowman 
muscle  and  not  the  ciliary  is  involved  in  the  placing  and  in 
the  holding  in  position  of  the  crystalline  lens.  Sa\age  (Oph- 
thalmic Myology,  2d  edition,  pp.  637-38)  discusses  in  a  very 
entertaining  manner  his  own  condition  of  astigmatism,  both 
from  the  ophthalmometric  and  subjective  testings  over  a  period 
of  years,  and  attributes  tiic  differences  in  large  measure  to  the 
lenticular  astigmatism  prdduced  by  a  tilting  of  the  crystallitic 
lens. 

(6)  Spherical  aberration.  In  (|uitc  hi.yh  degrees  of  astig- 
matism the  images  of  the  mires  are  affected  by  spherical  aber- 
ration. ( )n  this  account  there  is  recorded  an  excess  of  error. 
Leroy  and  Keid  insist  that  a  proper  reduction  in  the  astigma- 


OPHTHALMOPATHY  305 

tism  measured  by  the  ophthalmometer  must  be  made  if  it  is  to 
accord  with  that  found  on  subjective  examination. 

(7)  Contraction  of  the  recti  muscles.  By  voluntary  action 
the  recti  muscles,  in  a  few  cases,  alter  the  corneal  astigmatism. 
Davies  (Essay  on  Refraction  and  Accommodation  of  the  Eye, 
American  Encyclopedia  of  Ophthalmology,  Vol.  XIV,  page 
11168)  reported  in  1895  the  case  of  a  patient  who  had  a  corneal 
astigmatism  with  the  rule  of  0.5D.  which  could  be,  by  volun- 
tary action  of  the  recti  muscles,  increased  to  2D.  in  the  right 
eye  and  to  1.5D.  in  the  left  eye.  Under  a  cycloplegic  he  could 
still  increase  the  astigmatism  in  the  right  eye  to  1.5D.  and  in 
the  left  eye  to  ID. 
Ophthalmopathy.     The  pathology  of  the  eye. 

Ophthalmophthisis.     Wasting  of  the  eyeball. 

Ophthalmoplegia.    Paralysis  of  the  muscles  of  the  eye. 

Ophthalmoptoma.     Protrusion  of  the  eyeballs. 

Ophthalmorrhagia.     Bleeding  from  the  eyes. 

Ophthalmorrhexis.    Rupture  of  the  eyeball. 

Ophthalmoscope.  An  instrument  devised  by  Helmholz  for  ob- 
taining a  view  of  the  fundus  of  the  eye.  It  consists,  in  its 
essential  construction,  of  a  concave  mirror  with  a  central  peep- 
hole.    By  means  of  the  mirror,  light  is  thrown  into  the  eve 


Loring   Ophthalmascope. 


306  OPHTHALMOSCOPY 

and  on  tu  the  retina,  and  as  the  light  emerges  from  the  pa- 
tient's e3e  the  observer's  eye  is  interposed  in  the  path  of  the 
emergent  rays  by  being  placed  behind  the  mirror  at  the  peep- 
hole. 

The  modern  ophthalmoscope  is  furnished  with  a  self-illum- 
inating electric  bulb  and  battery,  and  fitted  with  a  set  of  re- 
volving lenses  which  can  be  wheeled,  at  pleasure,  between  the 
observer's  eye  and  the  patient's. 

OPHTHALMOSCOPY. 

Ophthalmoscopy,  as  its  name  implies,  is  an  objective  method 
of  viewing  the  interior  of  the  eye.  Its  principal — indeed,  we 
might  almost  say  its  only  practical  use  is  for  the  detection  of 
pathological  conditions  of  the  eye,  w  hich  manifest  themselves 
in  altered  appearances  of.  the  retina,  choroid  or  optic  disc,  as 
seen  through  the  ophthalmoscope.  It  can  be  utilized  to  de- 
termine the  refraction  of  the  eye,  but  its  employment  for  this 
purpose  is  so  difficult,  and  so  susceptible  of  error,  and  there 
are  so  many  and  so  much  superior  ways  of  estimating  refrac- 
tion, that  nobody  in  his  senses  would,  in  these  days,  think  of 
applying  ophthalmoscopy  to  this  form  of  work. 

THE   OPHTHALMOSCOPE. 

The  ophthalmoscope  was  designed  by  Helmholz  in  1845. 
Under  ordinary,  unaided  conditions  it  is  impossible  for  one 
person  to  see  another's  retina,  because  when  the  two  pupillary 
apertures  (observer's  and  observed's)  are  directed  toward  each 
other,  neither  can  be  a  source  of  light  to  the  other,  and  no  light 
can  pass  between  them.  It  is  for  this  reason  that  a  person's 
pupil  always  appears  black.  The  ophthalmoscope  was  designed 
to  furnish  means  whereby  light  may  be  thrown,  by  reflection, 
into  the  subject's  pupil  and  on  to  jiis  retina,  thus  illumining 
the  retinal  ground,  and  the  light  emerging  from  the  retina 
thus  illumined  may  be  intercepted  by  the  observer's  eye. 

It  is  really  a  very  simple  instrument.  It  consists,  essentially, 
of  a  small  concave  mirror,  pierced  through  the  center  with  a 
tiny  sight-hojp,  and  mounted  on  a  stem  haiidlo.  By  placing 
a  bright  source  of  light  just  to  one  side  of  the -patient's  head, 


OPHTHALMOSCOPY  30r 

on  a  level  with  the  top  of  his  ear,  and  holding  the  mirror  so 
as  to  front  his  pupil,  on  a  horizontal  line  with  the  pupillary 
center,  the  light  can  be  thrown  by  the  mirror  straight  into 
the  pupil  and  on  to  the  retina.  If  the  observer's  eye  be  now 
placed  behind  the  mirror,  thus  held,  so  that  he  looks  through 
the  sight-hole,  the  light  emerging  from  the  patient's  eye  will 
pass  directly  through  the  sight-hole  of  the  mirror  and  be  inter- 
cepted by  the  observer's  eye,  and  a  view  of  the  patient's  retina 
thus  obtained. 

From  the  rather  crude  instrument  originally  devised  by 
Helmholz  and  the  modern  ophthalmoscope  is,  of  course,  some- 
what of  a  far  cry.  There  have  been  no  changes,  however,  in 
the  essential  principle.  In  the  modern  instrument,  intended 
for  use  with  an  independent  source  of  light,  the  small  mirror  is 
usually  hinged  upon  a  perpendicular  swivel-pin,  which  enables 
it  to  be  turned,  laterally,  to  any  required  angle,  so  as  to  facili- 
tate focussing  the  reflected  light  upon  the  patient's  pupil.  A 
still  later  and  better  development  is  the  manufacture  of  an 
ophthalmoscope  having  its  own  source  of  light  in  the  shape  of 
a  tiny  high-power  electric  globe  set  at  the  top  of  the  stem 
handle,  immediately  in  front  of  the  mirror  at  such  an  angle  as 
to  be  horizontally  reflected  by  the  mirror.  An  arrangement 
for  sliding  the  lamp  up  and  down  makes  it  further  possible 
to  change  the  focal  relations  of  the  lamp  and  the  mirror,  so 
as  to  vary  the  size  and  intensity  of  the  illumination.  This  self- 
illuminating  instrument  does  away  with  the  trouble  of  focus- 
sing the  disc  of  light  upon  the  patient's  pupil,  and  makes  it 
possible  to  use  the  ophthalmoscope  anywhere,  in  any  position. 
The  lamp  is  fed  by  a  dry  battery  hidden  in  the  handle. 

Moreover,  all  ophthalmoscopes  of  modern  make  are  fur- 
nished with  a  revolving  battery  of  plus  and  minus  lenses,  set 
into  a  revolving  disc  behind  the  mirror,  so  that  any  desired 
lens  power  can  be  wheeled  into  place  behind  the  sight-hole  for 
observation  purposes.  This  arrangement  obviates  the  cumber- 
some necessity  of  placing  lenses  in  a  trial  frame  before  the 
patient's  eye. 

DIRECT    METHOD. 

There  are  two  methods  of  technique  in  the  use  of  the  oph- 
thalmoscope, each  having  its  special  uses  and  advantages.   The 


308  OPHTHALMOSCOPY 

direct  method  is  as  follows:  Having  thrown  the  light  into 
the  patient's  pupil,  we  gradually  approach,  with  the  mirror, 
nearer  and  nearer  to  the  patient's  eye,  until  the  mirror  almost 
touches  his  face,  and  look  directly  at  his  retina.  There  are  two 
or  three  conditions  which  must  be  observed  in  order  to  carry 
this  out  successfully.  Both  the  patient's  and  the  observer's 
eyes  must  be  emmetropic — if  not  naturally  so,  then  they  must 
be  rendered  emmetropic  by  each  wearing  his  proper  correction ; 
and  both  patient  and  observer  must  thoroughly  relax  their  ac- 
commodation. Under  these  conditions,  the  patient's  eye  emits 
neutral  waves,  and  the  observer's  eye  is  adapted  to  receive 
and  focus  neutral  waves. 

These  conditions  are  not  always  easy  to  realize.  It  is  espe- 
cially difficult  for  the  beginner  in  ophthalmoscopy  to  relax 
his  accommodation  when  looking  into  a  patient's  eye.  He  is 
almost  instinctively  impelled  to  accommodate.  Only  by  con- 
tinued practice  can  he  overcome  this  impulse.  He  must  culti- 
vate the  habit  of  imagining  that  the  retina  he  is  looking  at  is 
away  at  the  back  of  the  patient's  head.  As  soon  as  the  condi- 
tions are  met,  the  details  of  the  eye-ground  will  spring  into 
view. 

By  the  direct  method  we  obtain  an  enlarged,  upright,  virtual 

image  of  the  fundus,  with  very  clear  detail,  but  we  cannot  see 

more  than  a  portion  of  the  field  at  a  time,  so  that  we  must 

move  the  mirror  from  one  focal  plane  to  another,  examining 

the  various  parts  of  the  eye-ground  successively.     Its  great 

advantage  is  the  clear  detail  it  affords  us,  and  it  is  therefore 

the  best  method  for  examining  conditions  in   which   detailed 

pathology  is  of  importance.     The  areas  of  the  eye-ground  are 

magnified,  by  this  method,  about   14  diameters. 

1 
INDIRECT  METHOD. 

By  the  indirect  nutliod  we  hold  the  t)plUhalnU)SCope  about 

an  arm's  length   fnmi   the  patient's  eye,  throw   the  light  into 

his  pupil,  and  lujld  an  objective  convex  lens  of  about  6  cm. 

-focus,  i.  e.,  about  16  1).  in  front  of  his  eye,  between  it  and  the 

mirror.      The   distance    between    the   objective    lens   and    the 


OPHTHALMOSCOPY  309 

mirror  should  now  be  adjusted — by  moving  either  the  lens 
or  the  mirror,  or  both,  taking  care  to  keep  them  horizontally 
aligned — until  a  focussed  image  of  the  patient's  eye-ground 
comes  into  view.  It  is  a  much  easier  technique  than  by  the 
direct  method. 

By  the  indirect  method  we  get  a  real,  inverted  image  of  the 
eye-ground, — an  aerial  image  made  at  the  focus  of  the  objec- 
tive lens — enabling  us  to  see  practically  all  the  fundus  at  once, 
but  not  in  very  great  detail,  as  it  is  magnified  only  about  4 
diameters.  Its  great  advantages  are  the  ease  of  its  technique, 
and  the  perspective  view  it  affords  us  of  the  fundus,  showing 
us  the  relations  of  the  various  landmarks  to  each  other — al- 
ways bearing  in  mind,  of  course,  that  it  is  an  inverted  image, 
reversed  in  every  meridian. 

THE   EYE-GROUND. 

The  combination  of  structures  seen  at  the  back  of  the  eye 
by  means  of  the  ophthalmoscope  goes  by  the  name  of  the 
eye-ground,  or  fundus.  It  includes  the  blood-vessels  of  the 
retina  (the  retina  itself,  being  transparent,  is  not  seen  in 
health),  the  vessels  and  pigment  of  the  chorioid,  the  macula 
lutea,  and  the  optic  disc  or  nerve-head. 

The  normal  color  of  the  healthy  fundus  is  a  light  reddish 
yellow,  or  orange,  due  to  the  admixture  of  yellow  color  from 
the  choroid  pigment  and  red  from  the  blood  vessels.  It  is 
impossible  to  write  a  description,  or  to  present  a  colored  plate, 
or  even  a  series  of  colored  plates,  which  will  adequately  cover 
all  the  variations  in  quality  and  range  of  shade  that  are  in- 
cluded in  the  normal  color  of  the  eye-ground. 

In  blondes,  the  choroid  pigment  is  exceedingly  light;  hence 
the  fundus  is  correspondingly  light,  almost  straw-colored,  or 
perhaps  a  light  vivid  yellow.  In  brunettes,  the  pigment  is  dark 
brown ;  therefore  the  eye-ground  is  proportionately  dark.  Dif- 
ferent nationalities  exhibit  different  colors,  chief  among  which 
are  the  Southern  races,  who  have  a  characteristically  deep, 
rich  yellow  tint,  the  Orientals,  who  have  a  peculiar  stippled 
condition,  easily  mistaken  for  choroiditis,  and  the  Negroes, 
whose  funduses  are  almost  dark  grey  in  hue,  because  of  the 


310  OPHTHALMOSCOPY 

interjection  of  their  characteristic  dark  pigment.  With  all  of 
these,  and  other,  types  of  normal  coloration  the  operator  must 
make  himself  familiar. 

Perhaps  the  commonest  error  made  by  beginners  is  to  mis- 
take an  exceedingly  pale  fundus  for  anemia,  or  a  rather  deeply 
red  one  for  retinitis.  The  truth  is,  neither  anemia  nor  retinitis 
should  be  diagnosed  upon  the  general  coloration  of  the  fundus, 
but  upon  the  changes  which  take  place  in  these  conditions  in 
the  appearance  of  the  vessels,  which  is  described  below. 

Albinism.  In  some  persons  there  is  a  congenital  absence  of 
pigment  matter  in  the  iris  and  choroid.  (These  two  struc- 
tures, it  will  be  remembered,  are  parts  of  the  same  tunic.)  In 
such  cases  the  color  of  the  fundus  is  a  light  pink,  and  the 
vessels  of  the  choroid  are  very  conspicuous.  Other  signs  of 
albinism — milky  skin,  white  hair,  etc. — coexist. 

THE  MACULA. 

About  iy2  disc  diameters  to  the  temporal  side  of  the  fundus 
is  a  small  area  devoid  of  all  vessels,  in  the  center  of  which  is 
a  little  spot  of  darker  red  than  the  rest  of  the  eye-ground.  This 
is  the  macula  lutea,  or  yellow  spot.  Its  actual  size  and  color, 
like  the  color  of  the  eye-ground,  vary  in  different  individuals. 
It  is,  of  course,  exceedingly  sensitive  to  light,  and  the  patient 
cannot  stand  the  focussing  of  the  ophthalmoscope  upon  this 
spot  f(jr  more  than  a  moment  at  a  time. 

THE  NERVE  HEAD,  OR  OPTIC  DISC. 

A  little  to  the  nasal  side  (by  the  indirect  method,  of  course, 
to  the  opposite  side)  will  be  seen  the  head  of  the  optic  nerve, 
known  as  the  optic  disc.  It  appears  as  a  small  circular  disc  of 
considerably  lighter  color  than  the  surrounding  eye-ground — 
in  health  a  yellowish-pink,  with  a  soft  lustrous  texture.  Its 
actual  diameter  is  1..^  mm.,  l)ut  uiider  the  magnification  of  di- 
rect ophtl.almoscopy  it  appears  to  be  about  _'  cm.  in  diameter. 

Normally  the  surface  of  the  disc  is  tlush  with  that  of  the 
fundus,  and  can  l)e  focussed  simultaneously  with  the  fundus. 
Under  certain  conditions,  however,  it  is  depressetl  below  the 
level  of  the  fundus,  so  that  it  cannot  be  simultaneitusiy 
focussed.     This  contlition  is  known  as  "cupped  disc"  und  may 


OPHTHALMOSCOPY  311 

occur  without  the  existence  of  any  disease,  in  which  case  it  is 
called  "physiological  cupped  disc" ;  or  it  may  be  due  to  disease, 
either  to  optic  atrophy  or  to  glaucoma.  It  is  highly  important, 
therefore,  that  the  practitioner  be  able  to  distinguish  between 
a  physiologic  cupped  disc  and  a  pathologic  one. 

The  principal  indication  that  a  cupped  disc  is  physiologic 
is  the  absence  of  any  other  pathological  conditions.  The  sec- 
ond is  the  fact  that  it  is  only  partial,  i.  e.,  it  involves  only  a 
portion  of  the  disc,  usually  the  central  portion.  Add  to  this  a 
normal,  healthy  appearance  of  the  disc  itself,  with  the  vessels 
riding  smoothly  and  gradually  over  the  edge,  and  the  diagnosis 
of  a  physiological  cupped  disc  is  complete. 

ABNORMALITIES  OF  THE  DISC. 

Depressions.  Reference  has  already  been  made  to  the 
physiologic  cupping,  or  excavation,  of  the  disc.  Total  excava- 
tion is  practically  always  pathologic,  due  either  to  optic  atrophy 
or  to  glaucoma.  As  between  these  two,  the  distinction  is  often 
hard  to  make.  In  atrophy  the  cupping  is  shallow,  as  is  shown 
by  the  absence  of  parallax  motion  on  moving  the  ophthalmo- 
scope slightly  from  side  to  side ;  the  disc  itself  is  dead  white ; 
and  the  vessels  are  indistinguishable.  In  glaucoma,  the  exca- 
vation is  deep,  with  abrupt  edges,  and  shows  marked  parallax 
motion ;  the  disc  is  a  gray  pearl  color ;  the  veins  are  full  and  the 
arteries  small. 

Elevation.  (Swollen  disc.)  This  is  just  the  opposite  of  the 
foregoing  condition.  The  veins  surrounding  the  disc  are  dark, 
full  and  tortuous,  the  arteries  small.  The  nerve  fibres  are 
swollen  and  opaque,  and  the  disc  raised  above  the  level  of  the 
fundus;  its  color  is  a  reddish-gray,  and  its  margin  indistinct. 
This  condition  is  known  as  optic  neuritis,  papillitis,  or  choked 
disc,  and  always  indicates  grave  intra-cranial  disease.  Later 
on,  the  strangulation  of  the  nerve-head,  cutting  off  its  nutri- 
ment, produces  optic  atrophy,  and  blindness. 

Atrophy.  In  any  form  of  atrophy  the  appearance  of  the  disc 
is  exceedingly  white.  Its  exact  color  varies  greatly,  depending 
on  many  circumstances, — chalky,  snowy,  pearly,  bluish,  green- 
ish, etc.  But  always  it  is  exceedingly  white,  and  has  lost  the 
soft,  healthy  lustre  of  the  normal  disc.     Generally  speaking. 


312  OPHTHALMOSCOPY 

atrophy  is  either  primary,  i.  e.,  due  to  a  disease  of  the  optic 
nerve  itself,  or  secondary,  i.  e.,  due  to  strangulation  by  optic 
neuritis.  In  the  first  type  the  color  is  a  brilliant  pearly  white, 
the  edges  are  sharp-cut  and  dark,  the  lamina  cribrosa  (see 
Anatomy)  very  prominent,  and  the  vessels  almost  normal. 
In  secondary  atrophy  the  color  is  a  dirty  greyish-white,  the 
edges  indistinct,  the  lamina  not  conspicuous  and  the  vessels 
are  narrow  and  tortuous. 

THE  VESSELS. 

The  retinal  vessels,  arteries  and  veins,  can  be  seen  entering 
the  eye  around  the  margin  of  the  optic  disc,  and  distributing 
themselves  in  a  radiating  fashion, — principally  in  a  north  and 
south  direction  from  the  disc — over  the  eye-ground.  The 
choroidal  vessels  can  also  be  seen  through  the  retina  dis- 
tributed over  the  choroid.  The  distinguishing  points  between 
the  retinal  and  choroidal  and  retinal  vessels  are  as  follows: 

Choroidal —  Retinal — 

More  numerous  Not  so  numerous 

Larger  size  Smaller  size 

Close  together  Separated 

Nearly  parallel  Divergent  toward  periphery 

Frequently  anastomose  Do  not  anastomose 

Same  size  along  course  Diminish  in  size  toward  periphery 

Veins  and  arteries  same        \'eins  and  arteries  differ 
Flat  ribbon-like  appearance  Cylindrical  form 

Considering  the  retinal  vessels  alone,  we  have  to  distinguish 
between  the  arteries  and  the  veins.  In  health  the  differentiat- 
ing points  are  as  follows: 

Arteries —  \'eins — 

Bright  red  Dull  red 

Small  in  size  Large  in  size 

Light  streak  down  middle    Not  very   noticeable 
Course  fairly  straight  Course  rather  sinuous 

Do  not  pulsate  Frequently  pulsate 

Usually  cross  over  veins      I'sually  cross  under  arteries 

The  eye  is  the  only  part  of  the  body  where  the  veins  pulsate 
instead  of  the  arteries — i.  v.,  the  interior  of  tin-  e\e.  This  is 
due  to  the  incompressibility  of  the  vitreous,  which  causes  the 
pressure  of  the  arteries,  as  the  blood  is  pumped  into  them  by 


OPHTHALMOSCOPY  313 

each  heart-beat,  to  be  instantly  transmitted  to  the  veins,  forc- 
ing blood  out  of  them ;  as  soon  as  the  heart-beat  is  over,  and 
this  pressure  released,  the  blood  comes  back  into  the  veins, 
thus  giving  them  the  appearance  of  pulsation.*  The  phe- 
nomenon can  be  made  more  noticeable  by  exercising  gentle 
pressure  on  the  side  of  the  eyeball. 

The  pulsation  of  the  vessels  is  reversed,  i.  e.,  the  arteries 
pulsate  instead  of  the  veins,  in  any  diseased  conditions  which 
raises  intra-ocular  tension,  notably  in  glaucoma,  of  which  it  is 
one  of  the  important  symptoms. 

ABNORMALITIES  OF  THE  VESSELS. 

Hyperemia.  In  all  inflamed  conditions  of  the  retina  or 
choroid  the  vessels,  especially  the  veins,  become  enlarged  and 
engorged.  This  state  of  affairs  is  most  easily  observed  in  the 
region  of  the  disc,  for  here,  as  a  rule,  the  vessels  are  not  con- 
spicuous, but  Avhen  congested  they  thrust  themselves  upon  the 
observer's  notice  being  enlarged,  flattened,  and  deepened  in 
color.  This  is  a  much  more  dependable  sign  of  inflammation 
than  the  general  coloration  of  the  fundus. 

Special  forms  of  retinitis  and  choroiditis  will  be  described 
in  another  place. 

Anemia.  This  is  just  the  opposite  condition  of  the  fore- 
going. The  arteries  and  veins  are  equally  aft'ected,  hence  they 
retain  their  relative  sizes,  but  both  arteries  and  veins  are  much 
smaller,  often  indistinguishable,  and  less  red.  There  is  a  gen- 
eral pallor  of  fundus  and  disc,  but  the  lustre  of  the  latter  is  not 
lost. 

Sclerosis.  This  condition  pertains  to  the  arteries  alone.  They 
become  opaque,  lose  the  light  streak  down  their  middle  axes, 
and  in  advanced  sclerosis  become  altogether  white  themselves, 
due  to  reflection  of  light  from  their  opaque  walls.  The 
hardened  arteries,  pressing  upon  the  veins  where  they  cross 
them,  obliterate  them,  so  that  the  veins  appear  interrupted 
where  the  arteries  cross  them.  There  are  areas  of  pallor  over 
the  fundus,  where  the  nutriment  is  cut  oft'  by  the  obliteration 
of  the  vessels. 

*This  incomnressibility  of  the  vitreous  and  its  transmission  of  pressure 
to  the  veins  Is  similar  in  principle  and  mechanism  to  the  incompressibility  of 
water  which  gave  the  famous  depth-bomb  its  effectiveness  against  the 
submarine — transmitting  the  pressure  of  its  explosion  instantly,  through  the 
incompressible  water,   to  the  submarine,   causing  its   collapse  inward. 


314  OPHTHALMOSCOPY 

Embolism.  This  is  a  plugging  of  the  central  retinal  artery, 
or  one  of  its  branches,  by  a  blood-clot  floating  in  the  blood- 
stream. It  occurs  suddenly.  No  blood  gets  into  the  retinal 
arteries  supplied  by  the  plugged  vessel ;  hence  the  larger  vessels 
are  reduced  to  tiny  white  threads,  and  the  smaller  ones  vanish 
altogether.  There  is  an  area  of  pallor  around  and  adjacent  to 
the  disc,  representing  the  portion  of  the  fundus  deprived  of  its 
blood  supply.    There  is,  of  course,  sudden  and  total  blindness. 

Thrombosis.  This  rare  condition  is  essentially  the  same  as 
embolism,  except  that  the  plugging  of  the  artery  takes  place 
gradually,  the  final  obliteration  stretching  over  a  day  or  two. 
The  appearance  of  the  fundus  is  the  same. 

Hemorrhage.  In  certain  diseased  conditions — usually  those 
which  raise  the  blood  pressure  and  weaken  the  vessel  walls, 
the  retinal  arteries,  here  and  there,  give  way,  and  produce  a 
hemorrhage  on  to  the  retina.  Sometimes  there  is  one  large 
hemorrhage,  but  more  often  there  are  numerous  small  ones. 
The  ophthalmoscope  shows  a  cloudy  retina,  with  variously 
shaped  red  splashes  scattered  over  the  ground — round,  flame- 
shaped,  linear,  stellar,  and  irregular.  After  the  hemorrhages 
have  been  absorbed,  the  red  splashes  are  replaced  by  white 
spots. 

These  are  the  appearances  seen  in  cases  of  retinitis  due  to 
profound  systemic  diseases,  such  as  nephritis,  diai:)etes,  syphilis, 
malaria,  etc. 

Hyaloid  Artery.  Occasionally  the  hyaloid  artery,  which 
should  disappear  at  birth,  persists  through  life,  in  which  case 
it  api)ears  under  the  ophthalmoscope  as  a  dark  gray-colored 
thread,  attached  to  the  disc,  tloating  out  toward  the  vitreous, 
and  moving  with  rotation  of  the  eye-ball.  It  is  nt)t  a  patho- 
logical condition,  and  should  not  be  mistaken  for  one. 

Cilio-Retinal  Artery.  In  rare  instances  a  medium-sized 
artery  may  be  seen  running  from  the  temporal  edge  of  the  disc 
out  toward  the  macula.  This  vessel  is  known  as  a  cilio-retinal 
artery,  because  it  is  a  branch  of  the  ciliary  circulation  an- 
astomosing with  the  retinal  system.  In  retinal  embolism  the 
presence  of  this  anastomosing  artery  is  of  great  value,  as  it 
serves  to  kcej)  the  retinal  area  stijijilied  with  blood  in  spite  i)f 


OPHTHALMOSCOPY  3H 

the  blocking  of  the  central  retinal  artery,  thus  saving  the  pa- 
tient from  blindness. 

ABNORMALITIES  OF  THE  RETINA. 

Most  of  the  diseases  of  the  retina  are  in  reality  diseases  of 
the  vessels  supplying  the  retina,  and  have  therefore  already 
been  described  under  Abnormalities  of  the  Vessels.  However, 
if  a  vascular  disorder  of  the  retina  is  sufficiently  severe,  or  lasts 
long  enough,  it  brings  about  actual  pathologic  changes  in  the 
retina  itself.  The  principal  changes  to  which  the  retina  is  sub- 
ject are:  (1)  opacities,  (2)  exudations,  (3)  edema,  (4)  pigment 
changes,  (5)  detachment,  and  (6)   atrophy. 

All  of  these  conditions,  with  the  exception  of  pigment 
changes,  are  extremely  difficult  of  recognition  with  the  oph- 
thalmoscope by  any  except  an  expert.  Opacities  are  only 
recognizable  by  dint  of  our  inability  to  focus  certain  areas  of 
the  fundus  where  the  coat  is  opaque.  Exudates  and  edemas 
exhibit  a  grayish  color  of  the  fundus,  and  the  appearance  of  a 
gauzy  veil,  as  though  seen  through  a  fog. 

Pigment  Degeneration.  (Retinitis  Pigmentosa.)  This  con- 
dition is  more  easily  recognized.  The  pigmentation  appears 
in  masses  or  patches,  deposited  in  the  retina,  following  the 
course  of  the  retinal  vessels  and  often  forming  on  the  vessel 
walls.  They  are  often  star-shaped,  or  in  a  spider-like  forma- 
tion, giving  the  appearance  of  a  net-work  stretched  over  the 
fundus. 

Detachment.  Occasionally,  for  some  reason  (not  always 
clear)  the  retina  becomes  partially  detached  from  the  choroid, 
and  falls  over,  like  a  folded  curtain.  The  normal  reflex  is 
absent  at  the  detached  portion,  and  in  its  place  is  seen  a  gray- 
ish surface  of  uneven,  wavy  character. 

Atrophy.  Usually  accompanied  by  atrophy  of  the  choroid. 
The  white  sclera  is  seen  showing  through  as  white  patches. 

ABNORMALITIES  OF  THE  CHOROID. 

The  choroid,  being  a  vascular  coat,  like  the  iris,  is  much  more 
subject  to  inflammatory  diseases  than  the  retina. 

Choroiditis.  This  is  the  commonest  pathological  condition 
met  with  in  the  choroid,  and  there  are  innumerable  varieties  of 


316  OPHTHALMOSCOPY 

the  disease.  Simple  inflammation  and  hyperemia  of  the  chor- 
oid, and  even  hemorrhages  into  the  choroid,  are  difficult  of 
recognition  by  any  but  an  expert,  because  these  conditions 
are  concealed  by  the  retinal  coat.  It  is  not  until  the  inflamma- 
tion has  produced  deposits  and  degenerations  in  the  pigment 
that  it  becomes  easily  recognizable. 

At  this  stage  the  cardinal  ophthalmoscopic  symptom  con- 
sists of  patches  of  brown  or  black,  scattered  over  the  eye- 
ground,  which  presently  change  to  white,  as  the  exudates  are 
absorbed.  The  appearance  of  the  eye-ground  changes  from 
time  to  time,  new^  areas  of  pigment  degeneration  appearing, 
while  others  become  white.  Two  general  varieties  of  this 
form  of  choroiditis  are  recognized,  according  to  the  topography 
of  the  disease, — central,  in  w^hich  the  degeneration  lies  around 
the  disc,  and  disseminated,  in  which  the  patches  are  dissem- 
inated all  over  the  eye-ground,  especially  toward  the  periph- 
eries. Subjectively,  there  is  diminution  of  vision.  The 
trouble  is  usually  due  to  some  constitutional  disease,  most 
commonly  to  syphilis. 

Myopic  Choroiditis.  One  of  the  most  serious  types  of  chor- 
oiditis, and  one  which  especially  interests  the  refractionist,  is 
that  which  is  due  to  the  stretching  of  the  choroid  tunic  by  high 
myopia.  It  is  this  which  constitutes  the  grave  factor  in  what 
we  call  progressive  myopia. 

Posterior  Staphyloma.  This  is  not,  strictly  speaking,  a  con- 
dition of  the  choroid,  but  a  bulging  backward,  cone-fashion,  of 
the  sclerotic  coat  at  the  back  of  the  eye ;  but  it  is  the  result 
of  stretching  and  weakening  of  the  choroid.  It  is  recognized 
with  the  ophthalmoscope  by  the  different  power  necessary  to 
focus  the  l)ulged  portif)n  of  the  fundus. 

Coloboma.  Occasionally  there  is  a  congenital  absence  of  a 
portion  of  the  choroid  and  retina,  duo  to  arrest  in  develop- 
ment of  the  eye,  similar  to  that  which  is  .sometimes  seen  in  the 
iris,  as  though  a  piece  had  been  cut  out.  It  usually  appears 
in  the  form  of  a  triangle,  with  its  apex  at  the  disc.  The  white 
of  the  sclera  shows  strikingly  where  the  choroid  and  retina  are 
absent. 


OPHTHALMOSCOPY  317 

REFRACTION  BY  OPHTHALMOSCOPY. 

The  refraction  of  the  patient's  eye  can  be  roughly  estimated 
by  means  of  the  ophthalmoscope,  either  by  the  direct  or  by  the 
indirect  method. 

Direct  Method.  If  the  operator's  eye  wears  its  proper  dis- 
tance correction,  and  has  its  accommodation  thoroughly  re- 
laxed, and  if  the  patient's  accommodation  is  also  completely 
relaxed,  it  is  evident  that  the  only  other  factor  needed  to  obtain 
a  clear  view  of  the  patient's  fundus  is  that  his  eye  shall  be 
emmetropic,  i.  e.,  that  the  light  waves  emerging  from  it  shall 
be  neutral. 

If,  then,  under  the  above  conditions,  we  get  a  clear  view  of 
the  fundus,  we  conclude  that  the  patient's  eye  is  emmetropic. 
If  not,  he  has  an  error  of  refraction;  and  the  lens,  plus  or 
minus,  which,  when  wheeled  into  place  at  the  sight-hole,  gives 
us  a  clear  view  of  his  fundus,  is  the  measure  and  correction  of 
this  error.  If  there  is  astigmatism,  we  shall  find  that  it  requires 
different  lens  power  to  give  us  a  clear  view  of  the  fundus  in 
the  direction  of  the  two  chief  meridians. 

Indirect  Method.  Having  obtained  a  good  focussed  image 
of  the  optic  disc  by  means  of  our  objective  lens,  we  slowly 
move  the  objective  lens  away  from  the  patient's  eye  toward 
our  own,  taking  care  to  maintain  our  view  of  the  disc-image, 
and  note  whether  the  size  of  the  image  undergoes  any  change. 

If  the  patient's  eye  is  emmetropic,  i.  e.,  if  the  emergent 
waves  are  neutral,  then  it  is  evident  that,  no  matter  at  what 
distance  from  his  eye  the  objective  lens  is  held,  the  object  (i. 
e.,  the  patient's  disc)  will  be  situated  at  infinity  on  one  side 
of  the  lens,  and  the  image  will  be  situated  at  the  focal  length 
of  the  lens  on  the  other  side ;  and  the  image  will  therefore  re- 
main the  same  size  as  we  move  the  objective  lens  toward  us. 

If  the  patient  be  hyperopic,  i.  e.,  if  his  emergent  waves  be 
divergent,  then,  as  we  withdraw  the  lens,  increasing  the  dis- 
tance from  object  to  lens,  the  size  of  the  image  will  decrease 
inversely  in  reciprocals  of  focal  lengths.  The  image,  there- 
fore, will  seem  to  get  smaller  as  we  withdraw  the  objective 
lens  toward  our  own  eye. 

Again,  if  the  patient  be  myopic,  i.  e.,  if  the  waves  emerging 
from  his  eye  be  convergent,  then  the  object  focal  point  and  the 


318  OPHTHALMOSTAT 

image  focal  point  are  both  on  the  front  side  of  the  objective 
lens,  and  as  we  move  the  lens  forward  we  decrease  the  dis- 
tance from  object-focus  to  lens,  and  the  size  of  the  image 
increases  in  reciprocals  of  focal  lengths.  As  we  withdraw 
the  lens,  therefore,  toward  our  own  eye,  the  size  of  the  image 
appears  to  increase. 

If  the  eye  is  astigmatic,  the  change  in  size  of  the  image  will 
take  place  only  in  the  meridian  at  fault,  or  unequally  in  the 
two  chief  meridians. 

One  has  only  to  read  the  description  of  the  foregoing 
technique,  to  say  nothing  of  putting  it  to  trial,  in  order  to  see 
at  once  the  extreme  difficulty  of  fulfilling  all  the  necessary  con- 
ditions demanded  by  the  direct  method,  or  of  making  anything 
like  accurate  observations  by  the  indirect  method.  As  stated 
at  the  outset,  no  one  in  his  senses  would  nowadays  think  of 
employing  ophthalmoscopy  as  a  means  of  estimating  refrac- 
tion, in  the  face  f)f  the  difficulties  and  sources  of  error  involved, 
and  in  \iew  of  the  far  superior  procedures  at  his  disposal. 

Ophthalmostat.     An  eye  speculum. 

Ophthalmotosia.     Prt)trusion  of   the   eyeball. 

Ophthalmotropia.     Atrophy  of  the  eye. 

Ophthalmula.     A  scar  of  the  eyeball. 

Optic  Atrophy.  Atrophy  of  the  ojjtic  nerve,  with  partial  or  com- 
])lcte  loss  of  \  ision  as  a  result.  Under  the  ophthalmoscope  the 
nerve-head  (disc)  appears  as  a  sharply  defined  oval  of  dead 
white,  surrounded  with  a  dark  bordir.  It  is  a  hopeless  condi- 
tion.    (See  Ophthalmoscopy.) 

Optic  Axis.     .Sic  Axis. 

Optic  Commissure.    .Sec  Chiasm. 

Optic  Disc.     .See  Disc. 

Optic  Groove.     Si-c  Grove. 

Optic  Nerve.     .Sec  Optic  Tract. 

Optic  Papilla,     l^amc  as  Optic  Disc. 


Optic  tract  319 

Optic  Tract.  The  nerve  path  by  which  the  visual  impulses  travel 
from  the  retina  to  the  brain.  The  optic  nerve,  consisting  of 
fibres  gathered  up  from  the  two  lateral  halves  of  the  retina 
goes  backward  to  the  optic  chiasm,  where  the  fibres  from  each 
temporal  half  cross  over  to  the  other  side.  From  that  point 
the  lateral  half  of  each  retina  is  represented  on  the  correspond- 
ing side  of  the  brain.  Thence  the  tract  proceeds  backward  to 
the  quadregeminal  and  geniculate  bodies,  where  the  fibres 
terminate  and  deliver  their  impulses  to  a  new  set.  The  new 
tract  conveys  the  impulses  to  the  temporal  lobe  on  each  side 
of  the  brain,  which  is  the  center  of  pure  vision.  Thence  the 
impulses  are  relayed  by  another  set  of  nerve  fibres  to  the  left 
frontal  lobe  of  the  brain,  where  the  visual  impressions  are 
interpreted.     (See  Physiology.) 

Optical  Center.  The  point  on  the  principal  axis  of  a  lens  at  which 
the  secondary  rays  cut  the  principal  axis. 

Optician.  In  its  broad  sense,  this  word  signifies  a  person  who  is 
proficient  in  the  study  of  the  science  of  optics.  In  ordinary 
parlance,  it  is  usually  restricted  to  one  who  makes  optical  in- 
struments. 

Optics.  The  science  of  light  in  its  broadest  intent,  including  the 
physics,  mathematics,  and  physiology  of  the  subject. 

Optogram.  The  image  impressed  upon  the  retina  by  the  photo- 
chemical action  of  light. 

Optometrist.  One  who  measures  the  refraction  of  the  eye  and 
its  muscular  functions. 

Optometry.  The  science  and  art  of  measuring  the  refraction  and 
muscular  conditions  of  the  eye. 

Ora  Serrata.  The  anterior  border  of  attachment  of  the  retina 
to  the  choroid,  so  called  because  it  has  a  zig-zag  form  like  to 
teeth  of  a  saw. 

Orbicularis  Ciliaris.     See  Anatomy  of  the  Eye. 

Orbicularis  Palpebrarum.     See  Anatomy  of  the  Eye. 


320  ORBIT 

Orbit.  The  bony  socket  in  which  the  eyeball  rests,  formed  of 
borders  of  the  superior  maxillary,  frontal,  lachrj'mal,  palate, 
malar,  ethmoid  and  sphenoid.  The  orbit  is  shaped  somewhat 
like  a  cone,  the  apices  being  behind,  where  the  optic  nerve 
enters  the  eye.  The  extrinsic  muscles  are  all  attached  to  the 
orbit.     (See  Anatomy  of  the  Eye.) 

Orbital.     Pertaining  to  the  orbit. 

Orthochromatic.     Applied  to  vision  which  is  normal  for  colors. 

Orthometer.  An  instrument  for  determining  the  protrusion  of 
the  eyeballs. 

Orthophoria.  A  state  of  normal  function  and  balance  in  the 
extrinsic  muscles  of  the  eyes. 

Orthoptic.  Orthoscope.  Applied  to  methods  and  instruments 
for  the  purpose  of  correcting  heterophoria  and  strabismus,  by 
means  of  prisms. 

Orthoscopic  Lenses.  Combinations  of  lens  and  prism  so  adjusted 
that  accommodation  and  convergence  coincide. 

Orthotropia.  A  condition  in  which,  with  the  extrinsic  muscles 
in  passive  equilibrium,  there  is  normal  parallelism  of  the  visual 
axes  of  the  eyes.    Speaking  simply,  an  absence  of  squint. 

O.  D.    An  abbreviation  for  the  right  eye  (Oculus  dexter). 

O.  S.    An  abbreviation  for  the  left  eye  (Oculus  sinister). 

O.  U.    An  abbreviation  for  both  eyes  (Oculus  unity). 

Pachyblepharon.     Thickness  of  the  eyelids. 

Palpebra.     The  eyelid. 

Palpebral.     Pertaining  to  the  eyelid. 

Palpebral  Fissure.  The  opening  between  the  eyelids.  See  Eye- 
lids. 

Palpebritis.     Indaniination  of  the  eyelid. 

Pannus.  A  growth  of  false  tissue  under  llic  epithelium  of  the 
cornea,  due  to  continued  irritation,  as  \)\  tlic  granules  of 
trachoma. 


PANOPHTHALMIA  321 

Panophthalmia.  Panophthalmitis.  Inflammation  of  the  entire 
eye. 

Pantoscopic.  Applied  to  glasses  which  are  so  adjusted  as  to  give 
the  best  vision,  with  the  least  prismatic  effect,  in  all  directions. 

Papilla.  A  slight,  round  elevation  of  tissue.  In  the  eye  the  optic 
disc  is  often  called  the  papilla.  The  elevation  in  which  is  the 
opening  of  the  lacrymal  sac  is  called  the  lacrymal  papilla. 

Papillitis.    Another  name  for  optic  neuritis. 

Papillo-Retinitis.     Inflammation  of  the  retina  and  optic  nerve. 

Parablepsia.  A  condition  of  vision  in  which  objects  are  seen  dis- 
torted or  different  from  what  they  really  are. 

Farachromatism.    Color-blindness. 

Parachromatopsia.     Color-blindness. 

Paradoxical  Pupil.  A  condition  in  which  the  pupillary  reflex 
behaves  in  an  opposite  way  to  normal,  i.  e.,  the  pupil  expands, 
instead  of  contracting,  when  a  bright  light  is  thrown  into  the 
eye.    The  phenomenon  indicates  a  grave  disease  of  the  brain. 

Parallax.  The  apparent  change  of  place  which  objects  undergo 
by  being  viewed  from  different  points,  or  under  different  optical 
conditions. 

As  applied  to  lenses,  it  signifies  the  apparent  movement  of 
an  object  viewed  through  the  lens,  when  the  lens  is  moved  to 
and  fro  laterally  between  the  object  and  the  eye.  This  apparent 
movement  is  due  to  the  fact  that  a  lens  is  in  reality  a  series  of 
prisms  whose  bases  (in  convex  lenses)  or  whose  apices  (in 
concave  lenses)  meet  at  the  center  of  the  lens;  hence,  as  the 
lens  is  moved  before  the  observer's  eye,  so  that  he  no  longer 
looks  through  the  optical  center,  it  has  the  effect  of  a  prism, 
and  the  image  is  apparently  displaced  toward  the  apex  of  the 
prism.     (See  Prism.) 

In  the  case  of  a  convex  lens,  the  parallax  movement  is  in  the 
direction  opposite  to  that  in  which  the  lens  is  moved ;  in  con- 
cave lenses,  in  the  same  direction  that  the  lens  is  moved. 


322  -  PARALLAX  TESTS 

When,  instead  of  moving  the  lens  to  and  fro  between  the 
eye  and  the  object,  we  move  our  own  head  to  and  fro,  the  rela- 
tive movements  are  the  reverse  of  what  was  just  stated.  Or  if 
we  hold  the  lens  so  far  away  from  our  eye  that  we  are  outside 
of  its  principal  focus,  then  the  movements  will  be  reversed,  be- 
cause the  rays  reaching  our  eye  through  the  lens  are  reversed. 

Parallax  Tests.  I'ests  which  depend  upon  the  parallax  move- 
ments.   There  are  two  such  tests  common  in  ophthalmology : 

(1)  In  glaucoma,  when  the  optic  disc  is  cupped,  if  we  move 
the  ophthalmoscojje  to  and  fro,  laterally,  the  edge  of  the  disc 
and  the  central  portion  appear  to  move  at  different  rates  of 
speed,  owing  to  the  difference  of  level  between  the  edge  and 
the  bottom  of  the  cup. 

(2)  The  Duane,  or  cover  test  for  muscle  imbalance  is  also 
known  as  the  parallax  test.  (See  Heterophoria.)  We  can  ex- 
tend this  test,  after  we  have  determined  the  existence  of  im- 
balance, by  covering,  say,  the  left  eye,  and  having  the  right  eye 
fix  the  object,  and  then  suddenly  uncovering  the  left  eye.  If 
the  right  eye  remains  steady,  and  the  left  moves  into  position, 
the  trouble  is  a  functional  heterophoria.  If  the  right  eye  moves 
out  of  position  and  the  left  moves  into  })lace,  there  is  scjuint. 
and  the  left  eye  is  the  fixing  eye.  If  neither  eye  moves,  there 
is  squint  and  the  right  eye  is  the  fixing  eye. 

Parallelism.  A  state  of  being  parallel.  In  optics  the  term  refers 
to  that  state  of  the  eyes  in  which  the  \  isual  axes  are  ]>arallel. 
See  Convergence. 

Paralysis  of  Accommodation.  Paralysis  of  the  ciliary  muscle,  so 
that  accommodation  cannot  be  performed.  .*^uch  a  condition 
may  be  due  either  to  (a)  some  central  nervous  disease,  or  (b) 
poisoning  of  the  short  ciliary  nerves.  The  former  cause  is  ex- 
tremely rare.  The  sec(jnd  form  of  paralysis  we  bring  al)out 
artificially  when  we  put  atroi)inc  in  the  eye.  rathologically. 
it  is  almost  always  tin-  result  of  sonu-  infictious  disease,  such 
as  diphtheria,  scarlaliu.i,  typhoid,  etc. 

Parinaud's  Conjunctivitis.  Conjuui  tivitis  ch.iracleri/ed  by  red- 
dish j^rainilations  and  swclhn^  of  the  lyiupli  j^lands  in  the 
e.ir  and  throat,  s.aid  to  be  coiitr.icted  fioin  .iiiiuials. 


PAROPSIA  323 

Paropsia.    Same  as  Parablepsia. 

Patheticus.  A  name  given  to  the  superior  oblique  muscle  of 
the  eye.    (See  Muscles.) 

Pathologic.     Pertaining  to  disease. 

Pencil.  The  name  given  to  a  cone  or  cylinder  of  light  rays  con- 
taining all  the  color  waves  of  the  spectrum. 

Penumbra.  The  area  of  partial  shadow  between  the  full  light 
and  the  total  shadow  caused  by  an  opaque  body  intercepting 
the  light  from  a  luminous  body. 

Perception.  The  physiological  term  given  to  the  reaction  of  the 
brain  to  the  sheer  stimulus  of  a  sense-organ.  Thus,  perception 
of  light,  or  of  an  image,  is  the  sheer  knowledge  that  the  retina 
is  being  stimulated  by  light  or  by  an  image,  without  any  in- 
terpretation whatever.  It  does  not  even  include  projection  of 
the  stimulus  to  its  source  of  origin. 

Perichoroidal.    Surrounding  and  adjacent  to  the  choroid. 

Pericorneal.     Surrounding  the  cornea. 

Peridhoroidal.     Surrounding  the  choroid. 

Perimetry.  This  term  is  applied  to  the  measurement  and  out- 
lining of  the  visual  field,  by  whatever  means  it  is  carried  out. 
Although  the  pupillary  aperture  is  circular,  and  the  retina 
lies  in  a  segment  of  a  hollow  sphere,  the  stimulated  area  of  the 
retina,  on  which  the  light  falls,  and  therefore  the  contour  of 
the  visual  field,  (which  is  but  the  inverted  projection  of  the 
retinal  field),  are  by  no  means  circular.  The  retinal  field  does 
not  extend  equally  in  all  directions.  It  extends  furthest  toward 
the  nasal  side,  where  it  has  a  reach  of  90  degrees ;  hence  we 
can  see  objects  on  the  temporal  side  of  the  visual  field  as  long 
as  they  lie  on,  or  even  slightly  behind,  a  plane  passing  through 
the  pupil,  i.  e.  almost  at  right  angles  to  the  visual  axis  where 
it  is  tangent  to  the  cornea.  This  is  possible  largely  because  of 
the  strong  refraction  undergone  by  rays  falling  on  the  cornea 
at  this  angle.     Toward  the  temporal  side  and  downward  the 


324  PERIMETRY 

extent  of  the  retinal  field  is  much  less,  because  of  the  inter- 
position of  the  nose  and  the  brow,  cutting  ofif  the  entry  of 
light.  The  nasal  and  upper  ranges  oi  the  visual  field,  therefore, 
are  much  more  restricted  than  the  outer. 

The  actual  contour  of  the  field  may.  of  course,  be  varied  by 
the  way  in  which  the  head  is  tilted  during  the  observation ; 
however,  the  inner  field  is  never,  under  any  circumstances,  as 
extensive  as  the  outer.  Ordinarily,  the  measurement  and 
definition  of  the  field  is  done  with  the  head  in  a  perpendicular, 
fronting  position,  and  under  these  conditions  the  normal  con- 
tour of  the  visual  field  is  as  shown  in  the  accompanying  illus- 
tration. 

All  methods  of  measuring  the  \isual  field  consist  in  essen- 
tially the  same  process,  namely,  ascertaining  the  furthest 
peripheral  point,  along  each  of  the  meridians  of  the  eye,  at 
which  a  small  object,  entering  the  visual  field,  is  first  perceived. 
As  the  results  obtained  by  any  method  of  test  will  vary  with 
the  position  of  the  patient's  head,  the  size  of  the  object  used, 
the  degree  of  illumination,  and  other  circumstantial  conditions, 
it  is  important  that  these  conditions  be  well  adapted  to  the 
test  and  made  uniform  for  e\ery  examination. 

CONFRONTATION  TEST. 

For  clinical  purposes,  and  in  the  absence  of  instruments  of 
precision,  a  very  fair  rough  estimate  of  the  field  may  be 
obtained  l)y  the  use  of  the  practitioner's  hand  or  finger.  Prac- 
titioner and  patient  stand  or  sit,  face  to  face,  and  on  the  same 
le\el,  so  that  their  ])upils  front  each  other.  The  distance 
between  them  should  not  be  more  than  a  few  feet.  The  prac- 
titioner closes  his  right  eye  and  the  patient  his  left,  for  the 
examination  of  the  patient's  right  eye.  Both  practitioner's  and 
patient's  eye  are  now  in  precisely  the  same  relation  to  the 
visual  field  which  is  to  be  outlined. 

The  practitioner  now  i)uts  liis  li.md,  m  tiller,  out  laterally 
so  that  neither  hv  nor  tin-  patient  can  .sec  it,  and  gradually 
moves  it  inward,  toward  the  centre  of  the  lield,  instructing  the 
patient  to  s])eak  as  soon  as  it  becomes  visible.  This  is  repeated 
along  the  various  imridians  of  the  eye.  If  the  patiiiit  i)er- 
ceives   the   hand   at    substantiallv   the   same   inoineiit    thai    the 


PERIMETRY  325 

practitioner  does,  his  visual  field  in  that  meridian  may  be 
considered  normal ;  if  not  until  appreciably  later,  there  is  a 
contraction  of  the  field  in  the  meridian  in  question — hence  an 
abnormality  in  the  retinal  area  in  the  opposite  meridian. 

A  little  more  accurate  modification  of  this  test,  which  is 
known  as  the  confrontation  method,  can  be  obtained  by  sub- 
stituting- a  small  object,  such  as  a  tiny  square  of  cardboard, 
mounted  on  a  stem,  for  the  practitioner's  hand. 

Perhaps  a  still  more  accurate  and  convenient  method  is  that 
of  the  blackboard.  A  large  circle  and  its  meridians  can  be 
drawn  on  the  board,  the  patient  standing  a  few  feet  away, 
made  to  fix  his  eye  upon  the  centre  of  the  circle,  and  a  small 
piece  of  chalk,  for  the  object,  moved  inward  along  the  meri- 
dians, successively,  the  point  at  which  it  becomes  perceptible, 
being  marked  on  each  meridian,  and  these  points  being  joined 
up  by  a  line  at  the  completion  of  the  test.  By  this  means  a  very 
good  outline  of  the  visual  field  can  be  obtained. 

THE  PERIMETER. 

All  of  these  methods,  however,  have  one  objection,  namely, 
that  the  retinal  field,  being  concavely  spherical,  cannot  be 
accurately  projected  onto  a  plane  surface.  The  perimeter  is 
designed  to  remedy  this  objection,  and  at  the  same  time  to 
make  the  mechanical  features  of  the  test  more  accurate  and 
convenient.  There  are  several  varieties  of  perimeter  made, 
but  all  are  variations  of  the  same  principle. 

Essentially,  the  perimeter  consists  of  a  hollow  hemisphere 
disc,  either  whole  or  in  section,  mapped  out  into  meridians,  and 
the  meridians,  in  turn,  divided  into  short  equal  graduations, 
at  each  of  which  a  tiny  hole  pierces  the  disc.  There  is  a  head- 
rest for  the  patient's  head,  and  a  central  point  for  fixation.  As 
the  object  (usually  consisting  of  a  small  piece  of  card  or  tin 
on  the  end  of  a  stem)  is  brought  inward  along  each  meridian, 
and  the  patient  gives  notice  that  he  perceives  it,  the  point  of 
perception  is  marked  by  thrusting  a  pin  through  the  hole  in 
the  disc,  which  perforates  a  chart  inserted  at  the  back  of  the 
disc.  When  this  procedure  has  been  carried  out  for  all  the 
meridians,  the  perforations  on  the  chart  give  a  substantially 
accurate  outline  of  the  patient's  visual  field.  The  charts  usu- 
ally contain  a  printed  outline  of  a  normal  field,  so  that  the 


326  PERIMETRY 

pcrloratcd    outline    automatically    ccjiiipares    itself    with    the 
normal. 

THE  COLOR  FIELD. 

The  extension  of  the  retinal  and  \isual  fields  is  not  uniform 
for  colors.  The  peripheral  portions  of  the  retina  are  virtually 
color-blind,  so  that  when  a  colored  object  is  moved  gradually 
into  the  visual  field  it  is  first  percei\ed  merely  as  an  object 
without  color.  Not  until  it  is  moved  well  in  toward  the  central 
portion  of  the  field  does  its  color  become  perceptible.  Blue  is 
recognized  the  earliest;  that  is  to  say,  blue  has  the  largest  vis- 
ual field ;  green  has  the  smallest ;  red  is  somewhat  larger  than 
green ;  and  yellow,  again,  is  a  little  larger  than  red. 

Thus,  if  the  result  of  the  test  with  colors  be  normal,  the 
perforated  chart  will  show  the  outline  of  the  blue  field  on  the 
outside,  the  yellow  outline  inside  the  blue,  the  red  inside  the 
yellow,  and  the  green  inside  the  red  ;  all  of  them  substantially 
concentric  with  each  other.  Any  different  relations  will  indi- 
cate disturbance  of  the  retina. 

For  ordinary  purposes,  testing  with  black  and  white,  i.  c. 
with  a  white  object  on  a  black  field,  is  sufficient.  Tests  with 
colors  have  the  following  ad\antagcs:  (1)  They  are  more 
delicate  tests;  disclosing  retinal  defects  before  they  ha\  e  suf- 
ficiently jjrogressed  to  cause  any  loss  of  vision  for  black  and 
white,  (2)  They  give  information  concerning  the  color  sense 
itself,  and  (3)  There  are  certain  diseases  of  the  retina  which 
are  to  be  diagnosticated  by  distortions  of  the  color  field.  Con- 
spicuous among  such  diseases  is  toxic  amblyopia,  in  the  early 
stages  of  which  there  are  no  gaps  in  the  \  isual  field  when 
tested  with  black  and  white,  but  green  and  red  grow  \ery  indis- 
tinct (and  later  disappear  altogelher)  as  they  reach  the  centre 
of  the  field. 

CONTRACTIONS   OF   THE   VISUAL    FIELD. 

\\  hen  the  object  has  (o  be  lirouj^ht  ne.ircr  to  the  iciitti'  than 
normal  before  the  patient  perceixes  it.  the  lu-id  i>  said  to  be 
contracted  in  that  meridian,  (."ontr.iclioiis  of  the  liild  ,ire  of 
three  general  types:  (1)  Concentric,  in  which  iliere  is  a 
uniform  contraction  in  ;ill  meridians,  so  th.it  the  si/.e  of  the 
field   is   diminished,    but    its   contour   imchanged,    (2)    Sector- 


^  PERIMETRY  327 

shaped,  in  which  there  are  contractions  in  separate  meridians, 
making  triangular  cuts  into  the  contour  of  the  field,  and  (3) 
Hemiopic,  in  which  an  entire  half  of  the  field  is  wanting  (half- 
blindness). 

Concentric  contractions  of  marked  degree  are  usually  due  to 
hysteria  and  other  functional  nervous  troubles.  Sector-shaped 
contractions  are  invariably  due  to  organic  diseases  of  the 
retina.  Hemiopic  contractions  are,  of  course,  the  result  of 
lesions  which  impinge  on  the  optic  tract  where  one-half  of 
each  retina  is  represented,  i.  e.  from  the  optic  chiasm  to  the 
occipital  lobe  of  the  brain, 

SCOTOMATA. 

The  blind  areas  of  the  visual  field  represented  by  the  inter- 
ruptions under  the  second  variety  of  contractions,  due  to 
organic  diseases  of  the  retina,  are  technically  known  as  scoto- 
mata.  It  is  to  be  borne  in  mind  that  this  interruption  need  not 
necessarily  occur  at  the  peripheral  part  of  the  field,  i.  e.  on  the 
entry  of  the  object  into  the  visual  field.  As  the  object  is 
brought  inward  along  a  certain  meridian,  it  may  be  well  per- 
ceived at  the  normal  point  of  approach,  but  as  it  is  brought 
still  further  inward,  toward  the  centre  of  the  field,  it  may  go 
out  of  vision  again.  In  that  case  we  speak  of  the  blind  area 
as  a  central  scotoma. 

Subjectively,  scotomata  are  classified  as  positive  and  nega- 
tive. A  positive  scotomata  is  one  which  the  patient  himself 
perceives  as  an  area  of  darkness  projected  into  his  visual 
field.  A  negative  scotoma  is  one  whose  hiatus  in  the  visual 
field  is  not  perceived,  either  because  it  is  compensated  for  by 
the  vision  of  the  other  eye,  just  as  the  blind  spot  of  the  optic 
disc  is,  or  for  some  other  reason.  A  negative  scotoma  is  usu- 
ally not  discovered  until  an  examination  is  made  of  the  visual 
field  by  means  of  a  perimeter;  it  then  becomes  manifest,  just 
as  the  blind  spot  of  the  disc  does. 

INFLUENCE    OF    REFRACTION    ON    THE   VISUAL    FIELD. 

In  myopia,  with  its  relativel}-  long  eyeball,  the  extent  to 
which  the  rays  of  light  from  the  lateral  part  of  the  visual  field 
can  be  refracted  so  as  to  fall  upon  the  sensitive  portion  of  the 
retina  is  considerably  restricted.     In  myopia,  therefore,  there 


328  PERIOCULAR 

is  an  appreciable  concentric  contraction  of  the  visual  field.  In 
hyperopia,  with  its  shortened  eyeball,  precisely  the  opposite 
state  of  afifairs  exists;  hence  a  hyperope's  field  of  vision  is  con- 
centrically enlarged.  For  these  reasons  it  is  desirable,  iu  mak- 
ing examinations  of  the  visual  field,  to  have  the  patient  jjroperly 
corrected  for  distance  witii  a  sufficiently  periscopic  pair  oi 
lenses. 

Periocular.     Surrounding  or  adjacent  to  the  eye. 

Periophthalmic.     Same  as  Periocular. 

Periorbit.    The  parts  surrounding  the  orbit. 

Periorbital.     Surrounding  or  adjacent  to  the  orbit. 

Periorbitis.     Infiammation  of  the  periorbit. 

Periphacitis.  An  inllammation  of  the  tissues  surrounding  the 
crystalline  lens. 

Periphacus.     The  capsule  surrounding  the  crystalline  lens. 

Periphakitis.  Inllammation  of  the  parts  surrounding  or  adjacent 
to  the  crystalline  lens. 

Peripheraphose.  A  sensation  of  dark  spots  in  the  ])eripheral 
field  of  \ision. 

Periphery.  The  outer  margin  or  circumference  of  a  circle  or 
sphere.  By  extension,  the  part  of  a  system  furthest  remoxed 
from  its  functioning  centre. 

Peripupillometer.  .\n  instrunu-nt  for  measuring  tiie  I'xtent  of 
the  pupillo-m<jtor  area  of  the  retina  and  the  \ahie  of  the  pupil- 
lary reflex. 

Periscope.  .\ii  instrument  which,  by  means  of  prism  refraction 
and  minors,  enables  our  to  \  iiw  an  ol>ject  that  is  out?;ide  of 
his  line  of  \ision.  It  is  of  no  sjjecial  interest  to  the  refrac- 
tionist. 

Periscopic.  A  term  api)lie(l  to  a  lens  which  is  curved  to  conform, 
as  far  as  jx^ssible.  to  the  arc  of  rotation  of  the  eyeball,  so  that 


PERITOMY  329 

which  ever  way  the  visual  axis  is  directed  it  will  pierce  the 
lens  almost  perpendicularly.    See  Lens. 

Peritomy.  An  operation  for  pannus  (q.  v.)  by  cutting  a  strip  of 
conjunctiva  out  so  as  to  cut  off  its  nourishment. 

Perscorvinus.  A  w'rinkled  condition  of  the  inner  canthus,  com- 
monly known  as  crow's  foot. 

Perspicilium.    Any  instrument  which  improves  vision. 

Petit's  Canal.  The  space,  triangular  on  cross  section,  included 
between  tiie  fibres  of  the  zonula  and  the  equator  of  the  lens. 

Phacomalacia.    Soft  cataract. 

Phacometer.  An  instrument  for  measuring  the  curvature  of 
lenses,  determining  their  refractive  power,  and,  if  cylindrical, 
locating  their  axes. 

Phacosclerosis.     Hardening  of  the  crystalline  lens. 
Phacoscope.    An  instrument  for  viewing  the  crystalline  lens  and 
its  functioning. 

Phlyctenular  Conjunctivitis.  A  form  of  conjunctivitis  in  which 
there  are  tiny  blisters  or  blebs  on  the  membrane. 

Phlysis.     A  corneal  ulcer. 

Phoria.  The  position  of  the  eyeball  in  relation  to  its  visual  axis. 
In  common  optical  parlance  the  word  "phorias"  is  used  to 
indicate  the  various  types  of  muscular  imbalance. 

Phorometer.  An  instrument  for  detecting  and  measuring  im- 
balance of  the  extrinsic  ocular  muscles.  It  consists  essentially 
of  two  prisms  of  moderate  power — say  6  dioptres  each — geared 
so  that  they  can  be  rotated  simultaneously  in  opposite  direc- 
tions. The  prisms  are  first  set  with  their  bases  in,  so  that  the 
patient,  on  looking  through  them  at  an  object  six  meters  away, 
will  see  two  images,  each  image  on  the  same  side  as  the  eye 
which  perceives  it.  If  his  vertical  muscles  are  in  equilibrium 
the  images  will  be  on  the  same  level ;  if  he  have  right  hyper- 
opia, the  right  image  will  be  lower;  if  left  hyperopia,  the  left 


330  PHOSPHENE 

one  will  be  lower.  In  case  tiie  images  are  not  on  a  le\cl.  the 
prisms  are  rotated  until  they  do  so  stand  :  the  anicnuit  of  hyper- 
phoria can  then  be  read  off  on  the  scale. 

To  test  the  lateral  muscles  the  prisms  are  rotated  until  their 
bases  point  up  and  down,  respectively  ;  the  images  are  then 
doubled  vertically.  If  there  is  orthophoria,  they  stand  in  the 
same  \ertical  line ;  if  there  is  esophoria,  they  are  separated 
laterally  in  the  same  direction  as  the  eyes  which  perceive 
them  ;  if  exophoria,  they  are  separated  in  the  opposite  direc- 
tion. The  prisms  are  then  rotated  so  as  to  bring  the  two 
images  into  one  vertical  line,  and  the  amount  of  error  read  off 
on  the  scale. 

There  are  several  phorometers  on  the  market,  differing  from 
each  other  in  mechanical  principle  and  technique. 

Phosphene.  The  sensation  of  a  bright  border  around  the  visual 
field  during  a  sudden  relaxation  of  accommodation  in  the  dark, 
said  by  Czermak  to  be  due  to  dragging  on  the  retina  about  the 
ora  serrata. 

Pressure  Phosphene.  A  similar  sensation  caused  by  pressure 
on  the  eyeball. 

Photalgia.    T^iin  in  the  eye  produced  by  light. 

Photesthesia.     l^'xtreme  sensitiveness  to  light. 

Photogene.    A  prolonged  retinal  image. 

Photometer.  Photoptometer.  An  instrument  lor  testing  the 
sensitiveness  of  the  eye  to  light  by  determining  the  minimum 
illumination  under  which  an  object  becomes  visible. 

Photonosus.    .\ny  disease  of  the  eye  resulting  from  glare  of  light. 

Photophobia.  Intcjlerance  of  light.  This  condition  is  a  s\ mptoin 
in  ino>t  intlaniniatory  diseases  of  the  eye. 

Photophthalmia.  Any  disease  of  the  eye  due  to  exposure  to 
exiessixe  light,  such  as  snow-blindness,  lightning  stroke,  solar 
cataract,  etc. 

Photopsia.     A  subjective  sensation  of  light. 


PHOTOPTOMETRY  331 

Photoptometry.     Measurement  of  sensitiveness  to  light. 

Photoradiometer.  An  instrument  for  determining  the  power  of 
penetration  of  light. 

Phthisis  Bulbi.     Atrophy  of  the  eyeball. 

Physiology  of  Vision.  The  function  of  vision  may  be  divided 
into  three  phases,  or  stages:  (1)  The  mechanism  by  which 
light  waves  are  made  to  focus  upon  the  retina,  and  the  central 
rays  to  fall  coincidently  upon  the  two  maculae,  so  as  to  produce 
a  clear  and  single  image.  (2)  The  process  by  which  light 
energy  is  transformed  into  a  nerve  impulse  and  transmitted  to 
the  brain.  (3)  The  mental  process  by  which  the  image-picture 
so  produced  is  interpreted  and  acted  upon. 

(1)  The  first  phase  consists  of  (a)  the  refraction,  static  and 
dynamic,  of  the  eye,  and  (b)  its  motility.  The  static  refrac- 
tion of  the  eye  will  be  found  discussed  at  length  under  the 
heading  of  The  Eye,  and,  indeed,  forms  the  subject  of  many 
sections  of  this  book.  SufBce  to  say  here  that  the  cornea,  the 
aqueous  humor,  the  crystalline  lens  and  the  vitreous  together 
form  a  compound  lens  system  which  in  the  normal  eye  at  rest, 
is  exactly  adapted  to  focus  neutral  (infinite)  waves  upon  the 
retina. 

Dynamic  refraction  consists  of  an  active  efifort  on  the  part 
of  the  eye  to  increase  its  refractive  power  by  increasing  the 
convexity  of  the  crystalline  lens,  through  contraction  of  the 
ciliary  muscle,  in  order  to  focus  convergent  (finite)  waves 
upon  the  retina.  This  it  does  in  obedience  to  an  imperative 
desire  of  the  brain  for  a  clear  image.  The  process  will  be 
found  fully  treated  under  Accommodation,  and  the  nervous 
mechanism  by  which  it  is  accomplished  will  be  described  at 
length  later  in  this  article,  when  considering  the  eye  reflexes. 

The  motility  of  the  eye,  i.  e.  its  rotation  on  its  various  axes 
by  means  of  the  extrinsic  muscles,  is  for  the  purpose  of  direct- 
ing the  maculae  toward  the  object  looked  at,  so  as  to  ensure 
singleness  of  the  central  image.  This,  in  obedience  to  an 
imperative  desire  for  a  single  image.  These  movements  are 
of  two  kinds:  (a)  Conjugate,  where  the  two  eyes  are  moved 
in  the  same  direction,  to  the  right,  to  the  left,  up  or  down. 


332  PHYSIOLOGY  OF  VISION 

(b)  Coordinate,  where  the  two  eyes  are  turned  inward  or  out- 
ward. 

Conjugate  movements  up  and  down  are  performed  by  the 
pairs  of  superior  and  inferior  rectus  muscles,  respectively ; 
horizontally  bv  the  internal  rectus  of  one  eye  and  the  external 
rectus  of  the  other.  The  obliques  modify  all  these  movements. 
Coordinate  movements  are  performed  by  the  internal  recti  and 
external  recti,  respectively,  in  concert,  modified  by  the 
obliques.  A  full  discussion  of  these  latter  movements  will  be 
found  under  Convergence,  by  which  name  their  action  is 
known.  It  is  generally  assumed  that  there  is  a  special  cerebral 
centre  for  negotiating  the  act,  known  as  the  fusion  centre ;  its 
existence,  however,  is  not  proven. 

Coordinate  movements  inward  (positive  convergence)  are 
usually  accompanied  by  conjugate  movements  downward,  and 
coordinate  movements  outward  (negative  convergence)  by 
conjugate  movements  upward,  so  that  these  two  forms  of 
conjugate  movements  are  generally  regarded  as  part  of  the 
function  of  convergence,  of  which,  however,  they  are  not 
properly  a  part.  Occasionally  horizontal  conjugate  move- 
ments are  combined  with  positive  convergence ;  the  eyes 
looking  to  one  side  and  converging  at  the  same  time ;  and  this 
combination  is  a  little  difficult  of  explanation. 

It  is  rather  hard  to  see  how  the  internal  of  one  eye  and  the 
external  of  the  other  can  be  innervated  so  as  to  make  one  form 
of  movement,  and  the  internals  of  both  eyes  to  make  another 
form  of  movement,  siniultancously.  Nevertheless,  we  know 
that  they  are.  However,  the  combination  is  evidently  st)mc- 
what  of  a  nervous  strain,  for  the  power  of  convergence 
diminishes  in  proportion  as  the  eyes  are  turned  to  one  side. 
Indeed,  conjugate  movements  are  apparently  tiresome  acts, 
anyway,  which  the  individual  avoids  as  much  as  possible, 
preferring  to  turn  his  head  rather  than  his  eyes. 

(2)  Thus  far  the  eye  has  the  character  and  plays  the  part 
of  a  movable  and  adjustable  lens  or  camera.  The  real  physiol- 
ogy of  vision  begins  with  the  falling  of  the  focussed  waves  of 
light  upon  the  sensitive  film,  or  retina. 


PHYSIOLOGY  OF  VISION  333 

THE  OPTIC  NERVE  TRACT. 

The  optic  nerve  tract  consists  of  two  groups,  one  group  for 
each  eye,  of  several  thousand  neurons,  whose  cells  lie  distrib- 
uted over  each  retina,  in  the  rods  and  cones  of  Jacob's  mem- 
brane. Their  axis-cylinders,  in  each  case,  are  gathered  together 
in  a  bunch,  like  strands  of  silk  bound  into  a  rope,  surrounded 
by  a  strong  white  sheath,  to  form  the  optic  nerve,  and  travel 
backward  to  the  mid-brain.  In  each  eye,  the  axis-cylinders 
gathered  from  the  two  lateral  halves  of  the  retina,  i.  e.  the 
nasal  and  temporal  half,  remain  on  the  corresponding  side  of 
the  optic  nerve  until  some  inch  or  so  back  of  the  orbit,  where 
the  two  nerve  tracts  come  together  and  undergo  a  semi-cross- 
ing of  their  neurons. 

Here  the  neurons  from  the  inner  or  nasal  halves  of  the 
retinae  cross  over  to  the  opposite  sides,  while  those  from  the 


Fig.  1.  Showing  the  optic  tract.  Note  the  semi- 
crossing  of  the  neurons  at  the  chiasm,  so  that  each 
half  of  the  retinae  is  represented  on  the  same  side 
of  the  brain.  The  neurons  are  relayed  at  the  optic 
thalamus,  and  all  are  reunited  at  the  left  frontal 
convolution. 

outer  or  temporal  halves  remain  on  the  same  sides ;  so  that 
beyond  the  crossing-place  we  see  all  the  neurons  from  the  right 
half  of  each  eye  on  the  right  side  of  the  brain,  and  those  from 


334  PHYSIOLOGY  OF  VISION 

the  left  half  of  each  eye  on  the  left  side  of  the  brain.  The  place 
where  they  cross  is  called  the  optic  chiasm.     (Fig.  1.) 

After  this  crossing  (technically  known  as  decussation)  the 
two  tracts  travel  still  further  back  to  the  mid-brain.  Here  there 
is  an  important  sub-station,  consisting  of  several  groups  of 
nerve  ganglia,  among  them  the  geniculate  and  quadrigeminal 
bodies.  The  largest  of  them,  how'ever.  is  a  diamond-shaped 
ganglion  which  gives  its  common  name  to  the  entire  sub-sta- 
tion— the  optic  thalamus.  At  the  optic  thalamus  all  the  optic 
neurons  end,  and  deliver  their  impulses  to  a  new  relay  of  neu- 
rons. It  may  be  remarked,  in  passing,  that  all  the  neurons  of 
every  sensory  tract  in  the  body,  including  the  great  spinal 
tracts,  the  auditory  tract,  etc.,  arrive  at  this  same  junction 
place,  and  all  end  there  and  relay  their  impulses  to  fresh 
neurons.  It  will  therefore  appear  that  it  is  a  very  important 
sub-station   in   the  brain. 

From  the  optic  thalamus  the  optic  neurons  divide.  Some  of 
the  relay  neurons  go  backward  and  downward  to  the  fourth 
ventricle,  just  above  the  medulla  oblongata,  for  a  purpose  to  be 
related  presently  ;  others  go  back  to  the  occipital  lobe  of  the 
brain,  at  the  extreme  posterior  part,  where  the  centre  of  pure 
vision  is  located,  and  there  register  the  sensation  of  light. 
This,  it  is  to  be  understood,  is  the  case  with  the  tract  on  each 
side  of  the  brain. 

From  the  occipital  lobe,  the  impulse  is  again  relayed,  by 
another  set  of  neurons,  from  both  sides  of  the  head,  to  the  left 
frontal  lobe,  in  the  very  forepart  of  the  left  side  of  the  brain. 
to  the  centre  of  mind  \ision.  or  \isual  memory,  where  the 
image  is  perceived  and  interpretetl. 

This  gives  a  brief,  concise  account  of  ihe  \  isual  tract,  and 
the  course  of  the  visual  impulse  from  the  retina  to  the  brain, 
which  a  view  of  the  accompanying  cut  w  ill  hel[)  to  make  clear. 

It  is  evident,  from  the  abox  e  statements,  that  the  eye  is  not 
governed  bv  the  opposite  side  of  tiu-  brain,  as  is  tlie  arm  or 
leg.  Nor  is  it  governed  by  the  same  side,  as  are  the  face  and 
head.  Its  nervous  mechanisin  is  a  sort  of  mixture  of  direct  and 
opposite  control.  The  left  side  of  the  brain  is  represented  by 
the  right  half  of  the  cornea  and  lens,  and  the  left  half  oi  the 
retina,  in  each  eye;  the  right  side  of  the  brain,  by  the  left  half 


PHYSIOLOGY  OF  VISION  335 

of  the  cornea  and  lens,  and  the  right  half  of  the  retina,  in  each 
eve.  (It  Avill  be  remembered  that  the  light-waves  are  inverted 
between  the  cornea  and  the  retina.)  The  net  result,  however, 
is  that  the  right  brain  perceives  objects  on  the  left  of  the 
individual,  the  left  brain  objects  on  his  right. 

THE  VISUAL  IMPULSE. 

The  neurons  of  the  optic  nerve  are  light  neurons,  i.  e.  they 
are  designed  to  be  stimulated  by  light  vibrations  and  to  regis- 
ter in  the  brain  the  sensation  of  light.  Like  all  the  sensory 
nerves,  however,  they  are  capable  of  stimulation  by  almost  any 
kind  of  physiologic  stimulus,  but  register  always  the  same 
sensation.  Thus  we  can  stimulate  the  optic  nerve,  in  a  crude 
fashion,  by  means  of  heat,  or  cold,  or  pressure ;  but  the  sensa- 
tion produced  is  not  heat,  or  cold,  or  pain,  but  flashes  of  light. 

Normally,  however,  the  optic  nerve  is  stimulated  by  light. 
The  objects  of  the  outer  world  throw  their  images  on  the 
retina  ;  it  is  the  part  of  refraction  to  see  that  this  image  is  clearly 
focussed.  It  is  the  function  of  the  retina  to  transform  the  light 
vibrations  into  a  different  kind  of  vibrations,  viz.,  nervous 
excitations,  for  transmission  along  the  optic  neurons  to  the 
brain,  a  process  analagous  to  that  which  takes  place  in  the 
telephone,  where  sound  waves  are  transmuted  into  electrical 
waves  for  transmission  along  the  telephone  wires. 

The  place  at  which  this  transformation  takes  place  in  the 
eye  is  in  the  rods  and  cones  of  Jacob's  membrane.  The  precise 
manner  in  which  it  is  made  is  not  known.  That  remains  one 
of  nature's  secrets,  not  only  in  respect  of  the  retina,  but  of 
every  nerve-end.  We  know,  however,  that  most  of  the  energy 
of  the  light  vibration  is  used  up  in  producing  certain  chemical 
changes  in  the  rods  and  cones ;  hence  we  are  warranted  in 
concluding  that  these  changes  represent  the  physical  genera- 
tors of  the  nerve  current. 

It  was  a  German  physiologist  named  Boll  who  first  made  the 
important  discovery  that  the  retina  of  most  animals  (including 
man)  appears  of  a  purple-red  color,  and  that  during  life  this 
color  constantly  disappears  vinder  the  influence  of  light.  Fur- 
ther observation  showed  that  the  color  is  not  dependent  upon 
structural  relations,  but  is  due  to  a  pigment  which  is  decom- 
posed by  light.     This  pigment  is  called  the  visual  purple,  or 


336  PHYSIOLOGY  OF  VISION 

rhodopsin ;  and  it  is  the  decomposition  of  the  visual  purple, 
I)rincipally,  which  sets  up  the  nervous  stimulus  in  the  endings 
of  the  optic  nerve. 

The  visual  purple,  it  is  broadly  stated,  resides  only  in  the 
rods.  This  is  not  strictly  accurate;  for  there  are  cones  which 
contain  it.  As  a  general  proposition,  however,  the  statement 
may  stand,  that  the  rods  contain  visual  purple  and  the  cones 
are  devoid  of  it.  Every  color  wave  in  the  spectrum  has  the 
power  of  decomposing  the  visual  purjile — technically  known 
as  "bleaching."  because  under  decomposition  the  pigment  be- 
comes successively  yellow  and  white.  The  higher  the  color 
wave,  the  more  rapidly  it  decomposes  the  purple ;  violet  waves 
decompose  it  most  rapidly,  red  the  least.  In  the  dark,  or  dusk, 
the  visual  purple  remains  unchanged  ;  and  colors,  as  everyone 
knows,  are  not  perceived  in  semi-darkness,  although  line  and 
form  are  still  quite  recognizable. 

In  the  macula  there  are  no  rods,  but  only  cones,  and  no 
visual  purple.  In  this  area  the  sense  of  color  is  subordinated 
to  the  keen  discernment  of  line  and  form. 

After  being  decomposed  by  light,  the  visual  purple  is  regen- 
erated under  the  influence  of  darkness;  or  partial  darkness,  by 
the  epithelial  layer  of  the  retina. 

IMAGES  AND  THEIR  PROJECTION. 

Images  thrown  by  the  refracting  media  on  the  retina,  when 
they  are  well-defined,  leave  well-detincd  patterns  on  the  layer 
of  the  purple  rods — photographs,  as  it  were,  or  optograms.  It 
is  possible  to  make  these  optograms  on  the  retina  for  even  a 
short  period  after  death,  especially  in  lower  animals.  They 
can  be  produced  in  the  fresh  eyes  of  ral)bits  and  cattle. 

The  outlines  and  boundaries  of  these  patterns  du  llu-  relin.t 
determine  the  shape  and  form  of  the  image  as  perceived  by  the 
brain,  in  virtue  of  what  is  knt)wn  as  the  faculty  of  projection, 
i.  e.  the  i)ower  oi  the  brain  to  trace  back  every  sensation  along 
the  path  by  which  it  came  to  the  point  where  the  stimulus 
entered  the  body.  Thus,  in  the  cast-  of  a  retinal  im.-igr,  the 
brain  traces  back  the  coniposilc  sensation  made  by  this  p.it- 
tern-area  of  retinal  stiinulatidu,  along  each  separate  neuron, 
and  rccdiistructs  the  pattern. 


PHYSIOLOGY  OF  VISION  337 

As  a  matter  of  fact,  the  brain  traces  the  image-sensation  still 
further  than  that,  back  to  the  place  where  the  waves  of  light 
entered  the  eye,  i.  e.  to  the  cornea;  so  that  the  image  which 
the  brain  projects  and  reconstructs  is  the  corneal  image,  clari- 
fied by  refraction  and  visualized  by  the  retina.  This  simple 
fact — and  not  any  of  the  fantastic  theories  frequently  pro- 
pounded— explains  why,  with  an  inverted  retinal  image,  the 
brain  sees  objects  upright.  It  simply  projects  the  light  rays 
back  the  way  they  entered  the  eye. 

Projection,  so  far  as  it  is  an  integral  part  of  a  sense  itself, 
cannot  go  further  than  the  point  where  the  stimulus  entered 
the  body.  Therefore,  the  sense  of  vision,  of  itself,  cannot  pro- 
ject the  image  any  further  than  the  surface  of  the  cornea.  Its 
further  projection  out  into  space,  to  coincide  with  the  special 
position  of  the  object,  is  a  rather  complex  operation  of  the 
brain,  in  which  others  of  the  senses,  also  memory  and  expe- 
rience, play  a  considerable  part.  We  shall  have  more  to  say 
on  this  point  later.  It  is  this  projection  of  the  visual  image 
into  space  that  makes  vision  an  external  sense. 

THE  RETINAL  AND  VISUAL  FIELDS. 

The  area  of  the  retina  on  which  light  falls  from  the  outer 
world,  when  there  is  no  abnormal  interference,  is  called  the 
retinal  field.  This  field  is,  of  course,  limited  and  shaped  by 
the  anatomical  conditions  of  the  face  and  the  eye.  In  general 
its  angular  dimension  is  fixed  by  the  size  of  the  pupil,  which, 
as  the  nodal  point  lies  close  behind  it,  permits  a  very  wide 
visual  angle.  The  dimension  of  the  retinal  field  is  determined 
by  the  size  of  this  angle. 

The  shape  of  the  retinal  field  is  also  substantially  the  shape 
of  the  pupillary  opening,  except  on  the  nasal  side  of  the  retina, 
where  the  prominence  of  the  nose  cuts  oiif  a  portion  of  the 
light  from  entering.  The  actual  contour  of  the  field,  therefore, 
is  as  shown  in  the  accompanying  cut.     (Fig.  2.) 

Light  from  the  right  side  of  the  outer  world  falls  on  the 
left  half  of  the  retinal  field ;  and  vice  versa.  The  macula,  being 
always  at  the  retinal  extremity  of  the  visual  axis,  is  always 
at  the  centre  of  the  retinal  field.  At  this  place  the  acuity  is 
greatest  and  the  retinal  field  most  clearly  and  strongly  defined ; 


338 


PHYSIOLOGY  OF  VISION 


Fig.  2.  Showing  the  contour  of  the  visual  field  of  the 
left  eye.  Note  the  cut-off  due  to  interposition  of  the 
nose. 


from  there  out  to  the  circumference  the  rods  and  cones  become 
less  and  less  sensitive,  and  vision  less  and  less  distinct. 

The  retinal  field  is  outlined  l)y  means  of  a  device  called  the 
perimeter.  It  consists  of  a  large  concave  disc  with  a  central 
target  from  which  radiate  meridianal  lines.  While  the  subject 
fixes  the  central  target  with  his  eye.  a  small  object  (piece  of 
card  cm  a  long  thin  stick)  is  slowly  moved  from  the  circum- 
ference of  the  disc  inward  along  one  of  the  meridians  until  it 
comes  within  the  subjects  vision.  This  is  done  successively 
along  the  meridians,  a  mark  being  made  on  the  meridianal  line 
(or  on  a  chart  behind  it)  to  record  the  point  where  the  object 
first  became  visible.  These  marks  are  then  joined  up  all  round 
the  disc,  or  chart,  and  the  resulting  outline  gives  the  ct)ntour 
(jf  the  retinal  field.     (See  Perimeter.) 

The  visual  field  is  the  outer,  objectixc  complement  of  the 
retinal  field,  i.  c.,  the  area  in  the  outer  world  from  which  light 
falls  on  the  retina  and  stimulates  it.  The  right  visual  field  is 
represented  on  the  left  retinal  field,  ami  \  ice  versa.  Light  from 
the  centre  of  the  field  falls  on  the  macula.  Only  that  part  of 
the  visual  field  which  is  represented  on  the  macula  is  viewed 
with  attenti(jn  and  clearness;  for  only  there  do  the  images  fall 
on  identical  points  of  the  two  retinae,  and  only  there  is  the 
visual  acuity  sufficiently  developed  to  perceive  detail  (See 
"The  Visual  Impulse").  The  rest  of  the  field,  which  falls  on 
the  outer  portions  of  the  retina,  docs  not  fall  upon  identical 


PHYSIOLOGY  OF  VISION  339 

points,  and  is  therefore  seen  double ;  and  the  rods  and  cones 
are  not  sufficiently  sensitive  to  recognize  detail.  However,  the 
perception  of  this  part  of  the  field  is  good  enough  to  enable 
us  to  see  our  way  around,  and  to  call  our  attention  to  any 
object  which  we  wish  to  fix,  so  that  we  may  turn  our  maculae 
toward  it. 

Again,  owing  to  the  interruption  to  the  light  by  the  prom- 
inence of  the  nose,  and  to  the  fact  that  images  do  not  fall  on 
identical  spots  in  the  retinae  except  at  the  maculae,  one  part 
of  the  visual  field  (that  which  falls  on  the  maculae)  is  accur- 
ately superimposed — a  printer  would  say  it  "registers  true" — 
and  is  seen  singly  by  the  two  eyes  in  fusion  ;  other  portions 
overlap  in  the  two  retinae  and  are  seen  indistinctly  and  blurred ; 
still  others  (those  which  fall  on  the  parts  of  the  retinae  cut 
ofif  by  the  nose)  appear  only  on  one  retina  and  not  on  the 
other.  All  this  overlapping  and  unequal  distribution  of  the 
visual  field  on  the  two  retinae,  however,  does  not  bother  the 
vision,  because  it  afifects  only  the  outer  area  and  not  the  cen- 
tral vision.  As  soon  as  we  turn  our  maculae  toward  an  object 
to  view  it  attentively,  that  object  does  not  overlap. 

There  is,  moreover,  a  dark  area  of  no  vision  at  all  in  the 
visual  field  of  each  eye,  corresponding  to  the  small  portion 
which  falls  on  the  blind  spot  of  the  retina  (the  optic  disc)  ;  but 
as  this  dark  area  represents  a  different  portion  of  the  visual 
field  for  each  eye,  the  portion  that  one  eye  does  not  see  the 
other  one  does,  and  so  it  is  not  noticed. 

EYE  REFLEXES. 

In  describing  the  optic  nerve  tract  it  was  stated  that  at  the 
optic  thalamus  the  neurons  divide,  one  group  going  back  to 
the  centre  of  pure  vision  in  the  occipital  lobes  of  the  brain, 
another  group  traveling  downward  to  the  fourth  ventricle. 
This  latter  course  forms  the  path  of  what  is  known  as  the 
light  reflex,  for  bringing  about  contraction  of  the  pupil,  as  a 
protection  to  the  retina  against  excessive  light. 

When  light  is  thrown  upon  the  retina,  stimulating  the  rods 
and  cones,  the  impulse,  as  we  have  seen,  is  carried  backward 
along  the  optic  tract  to  the  optic  thalamus.  From  this  point, 
a  part  of  the  impulse  goes  to  occipital  lobes,  registering  the 


340  PHYSIOLOGY  OF  VISION 

sensation  of  vision,  and  part  to  a  centre  in  the  fourth  \entricle. 
just  above  the  meduHa  oblongata.  This  centre  is  practically 
a  spinal  centre,  serving  as  an  automatic  arc.  where  the  impulse 
is  transferred  to  a  motor  neuron,  the  third  cranial  nerve,  or 
motor  oculi,  whose  root  is  in  the  fourth  ventricle.  The  impulse 
is  carried  by  neurons  of  the  third  nerve  out  to  the  ciliary  gang- 
lion, where  it  is  delivered  to  the  ciliary  neurons  of  the  sympa- 
thetic, and  by  them  conveyed  to  the  concentric  muscles  of  the 
iris,  causing  them  to  contract  and  diminish  the  size  of  the 
pupil.  This  constitutes  the  light  reflex.  It  is  absolutely  auto- 
matic and  involuntary,  taking  place  with  unfailing  constancy, 
even  during  unconsciousness,  in  a  healthy  individual.  Its 
absence  always  indicates  a  grave  pathological  condition,  in 
which  the  nerve  path  is  blocked,  either  by  disease  or  by  drug 
poisoning. 

Another  important  reflex  of  the  eye  is  the  accommodation 
reflex,  for  the  purpose  of  contracting  the  ciliary  muscle.  The 
stimulus  in  this  case  is  the  falling  upon  the  retina  of  an  indis- 
tinct image;  the  impulse  is  carried  along  the  entire  optic 
tract  to  the  centre  of  mind  vision  in  the  left  frontal  brain  ; 
there  it  is  transferred  to  one  of  the  motor  projection  neurons 
of  the  brain  and  taken  down  to  the  fourth  ventricle,  relayed  to 
the  motor  oculi,  by  which  it  is  carried  to  the  ciliary  ganglion, 
and  (lelixered  to  the  ciliary  neurons  of  the  sympathetic;  these 
carry  it  to  the  ciliary  muscle,  and  cause  the  muscle  to  contract 
for  the  ])urpose  of  accommodation.     (I'ig.  3.) 

The  accommodation  reflex  is  not,  strictly  speaking,  an  auto- 
matic retlex.  since  its  transfer  arc  is  in  the  \()luiitar\    area  of 


I'iH-  '■'■  SUitwiuii  ihf  pitUi  t>(  llio  lu-cuiDiiiiiilalliiii  if- 
llt'X  (ill  iiiilit'okcii  lirK's).  mill  llial  nf  (hi-  IlKhl  rt-rifX 
(ill    lir<ik<-n    lliK-.s.) 


PHYSIOLOGY  OF  VISION  341 

the  brain  and  it  can  be  withheld  at  will.  One  does  not  have  to 
accommodate  in  response  to  an  indistinct  image,  as  he  has  to 
contract  his  iris  in  response  to  light.  However,  the  act,  as 
ordinarily  performed,  is  so  instinctive  and  unconscious,  in 
obedience  to  the  imperative  desire  of  the  brain  for  a  clear 
image,  that  it  is  generally  regarded  as  a  technical  reflex.  It 
occurs,  of  course,  only  during  consciousness ;  and  its  absence, 
like  that  of  the  light  reflex,  is  a  sign  of  serious  disease.  It  is, 
in  fact,  less  often  interrupted  by  disease  than  the  light  reflex. 

In  certain  nervous  diseases,  notably  locomotor  ataxia,  the 
light  reflex  is  abolished  while  the  accommodation  reflex  re- 
mains intact;  so  that  the  pupil  fails  to  contract  in  response  to 
light,  but  contracts  during  accommodation.  Such  a  condition 
is  known  as  an  Argyll  Robertson  pupil,  after  the  man  who 
demonstrated  it.  In  other  diseases,  the  pupil  responds  to  light 
but  not  to  accommodation,  and  is  then  known  as  a  Wernicke 
pupil. 

The  eyes  negotiate  several  other  so-called  reflexes,  none  of 
which,  however,  are  genuine,  automatic  reflexes,  but,  like  the 
accommodation  reflex,  take  place  through  the  intelligent  cen- 
tres of  the  brain,  so  instinctively  and  instantaneously  as  to 
partake  -of  the  nature  of  true  reflexes.  Of  these,  the  one  which 
approaches  most  nearly  to  a  true  reflex  is  the  act  of  winking 
when  an  object  is  brought  very  near  the  eye.  The  path  taken 
by  this  reflex  is  the  same  as  that  taken  by  the  accommoda- 
tion reflex,  except  that  it  travels  along  different  neurons  of 
the  motor  oculi  to  the  muscle  of  the  eyelid.  This  act  of 
winking  in  response  to  vision  must  not  be  confounded 
with  the  automatic  act  of  winking  which  is  kept  up 
continuously  for  the  purpose  of  cleaning  the  eyeball.  The 
latter  is  a  genuine  reflex,  which,  however,  has  nothing  to  do 
with  the  optic  tract.  Its  stimulus  is  the  irritation  of  the  con- 
junctiva and  cornea,  and  the  impulse  travels  by  way  of  the 
sympathetics  and  the  fifth  cranial  nerve  to  the  fourth  ventricle, 
where  it  is  transferred  to  the  motor  oculi. 

Other  pseudo-reflexes,  which  we  perform  every  hour  of  the 
day  in  response  to  stimulation  of  the  retina,  are  those  which 
are  transferred  at  the  centre  of  mind  vision  to  other  centres 


342  PHYSIOLOGY  OF  VISION 

and  given  lo  nioti^r  neurons  going  to  various  muscles  of  the 
body  ;  as  when  we  avoid  a  blow  which  we  see  coming,  or  get 
out  of  the  way  of  a  car.  or  laugh  at  a  funny  scene,  or  cry  at  a 
sad  one,  etc.  None  of  these  eye  reflexes,  except  the  light 
reflex,  is  a  genuine,  automatic  reHex,  and  the^'  occur  only 
during  consciousness. 

VISION  AND  EQUILIBRIUM. 
Mention  has  been  made  of  the  part  which  vision  plays  in  the 
faculty  of  co-ordination,  i.  e.,  of  maintaining  equilibrium.  We 
stand,  and  walk,  and  perform  other  muscular  acts,  continually, 
by  the  aid  of  our  vision,  which  keeps  us  advised  of  spacial 
relations.  In  the  use  that  we  habitually  make  of  it,  vision  is 
a  most  important  factor  in  co-ordination.  In  another  sense,  it 
is  not  nearly  so  important  a  factor  as  the  muscle  sense  and  the 
semi-circular  canals  of  the  ear ;  namely,  that  if  we  are  deprived 
of  vision  it  has  very  little  elYect  on  our  co-ordination  and 
equilibrium  ;  we  soon  learn  to  do  without  it.  The  other  two 
factors  we  could  not  do  without.  A  very  striking  example  of 
the  way  in  which  vision  controls  our  nervous  balance  is  to  be 
seen  in  the  different  way  in  which  a  sudden  shock  afTects  us 
according  to  whether  or  not  we  see  it  coming.  If,  groping 
around  in  the  dark,  we  suddenly  lay  our  hand  on  a  cold,  wet 
rag,  it  makes  us  jump:  but  if  we  had  seen  it  just  before  we 
touched  it,  we  should  recei\e  the  sensation  cpiite  calmly. 

PURE  VISION. 

The  first  ])lace  at  which  the  \  isual  impulse,  carried  back 
along  the  oi)tic  tract,  registers  a  sensation,  is  at  the  occipital 
lobe  of  the  brain,  in  the  very  back  of  the  heatl.  The  sensation 
registered  here  is  sheerly  that  of  ])erception.  l-.ach  lobe,  on 
either  side  of  the  brain,  jxrceives  half  the  \isual  field,  since  it 
receives  neurons  from  one-half  of  each  retina  ;  so  that  both 
must  be  functioning  in  order  to  see  a  complete  picture.  ( )cca- 
sionally  we  meet  with  a  disease  of  the  brain  which  ilestroys 
one  of  these  lobes,  and  then  the  patient  is  blind  to  one  side  of 
the  visual  field;  smli  .1  ionditinn  is  culled  henii.-iiidpsi.i,  or  lialf- 
blindness. 

If  the  imi)ulse  got  no  further  than  the  occipital  lobes,  i>r  if 
the   rest   of   the  itplic   tract,   beyond   the  occipital   lobes,   were 


PHYSIOLOGY  OF  VISION  343 

impaired,  vision  would  be  sheer  perception  and  nothing  more. 
We  should  see  objects,  to  be  sure,  but  they  would  have  no 
meaning  for  us,  nor  even  any  spacial  relations.  Nor  should 
we  ever  remember  what  we  saw ;  every  time  the  image  fell  on 
our  retina,  it  would  be  new  to  us.  Our  vision,  therefore,  would 
do  us  no  good.  Occasionally  we  meet  with  patients  who  are 
in  just  this  condition.  Delirious  people  are  temporarily  in  this 
state;  certain  forms  of  idiotcy  permanently  so.  Even  a  normal 
person  occasionally  experiences  a  fleeting  instance  of  it — for 
example,  just  as  he  awakens  from  a  sound  sleep,  until  he  "gets 
his  bearings."  In  order  that  the  perceived  picture  may  have 
any  meaning  and  value,  it  must  be  transmitted  to,  and  acted 
upon  by,  the  higher  brain  centres. 

Pure  vision,  then,  is  only  a  complemetary  part  of  vision — 
but  an  important  one.  Without  the  higher  centre  it  would  be 
without  any  meaning  or  value  ;  on  the  other  hand,  without  the 
centre  of  pure  vision  the  higher  centre  would  have  no  image 
to  interpret. 

MIND  VISION. 

The  impulses  received  by  the  occipital  lobes  are  relayed  by 
them,  along  fresh  neurons,  to  one  single  centre  in  the  left 
frontal  convolution  in  the  brain,  located  just  posterior  to  the 
fissure  of  Rolando,  a  few  inches  back  of  the  forehead.  This  is 
the  centre  of  mind  vision.  What  happens  here  is  a  very  com- 
plicated affair  which  would  take  volumes  to  discuss  fully.  We 
shall  discuss  it  very  briefly  and  generally. 

The  functions  of  this  higher  centre  of  vision  may  be  roughly 
divided  into  four,  although  each  of  the  four  is  itself  a  multiple 
and  complex  performance : 

(1)  Visual  Memory.  This  is  the  simplest  and  earliest  of 
the  higher  visual  functions.  It  consists  essentially  in  the 
storing  of  repeated  images,  until  the  brain  becomes  familiar 
with,  and  recognizes  them.  This  is  a  necessary  preliminary  to 
whatever  other  operations  the  brain  may  have  to  carry  out  in 
regard  to  its  images ;  and  it  is  this,  chiefly,  that  the  visual 
centre  of  the  infant  is  concerned  with  during  the  first  year  or 
so  of  life,  although,  to  a  less  extent,  we  continue  to  do  it  all 
our  lives. 


344  PHYSIOLOGY  OF  VISION 

Just  what  the  nature  of  this  memory  process  is,  it  is  impos- 
sible even  to  guess.  Most  likely  it  is  analagous  to  the  learning 
of  a  code  in  telegraphy,  each  kind  of  image  registering  a  special 
kind  of  impression,  which,  after  repeated  registering,  meets 
with  a  sort  of  automatic  response  in  the  brain  centre.  This, 
however,  does  not  explain  the  ])ower  of  the  brain  to  recall  the 
impressions  at  will,  witliout  any  stimulus  from  the  retina  at 
all,  which  forms  a  part  of  the  memory  faculty.  We  must  just 
leave  it  unexplained. 

(2)  Visual  Classification.  This  is  the  faculty  of  resolving 
images  into  their  attributes  and  qualities,  color,  form,  num- 
ber, etc.,  and  comparing  them  for  their  likeness  or  unlikeness. 
The  nature  and  mechanism  of  this  faculty  is  c|uite  beyond  any 
attempt  to  explain.  It,  also,  is  a  faculty  with  which  the  child 
is  very  busy  in  its  early  life. 

(3)  Visual  Judgments.  This  faculty  consists  in  comparing 
the  data  furnished  by  the  image  itself  with  the  data  derived 
from  others  of  the  senses,  touch,  muscle  sense,  etc..  and  thereby 
arriving  at  certain  conclusions  which  are  not  really  a  part  of 
the  function  of  vision  at  all.  but  so  intimately  bound  up  with 
it  as  to  be  generally  regarded  as  a  part  of  it;  the  size  of  the 
object,  its  distance  from  the  eye,  its  solidity.  The  way  in 
which  these  judgments  are  formed  will  be  further  discussed 
presently. 

(4)  Visual  Associations.  This  is  the  highest  and  most  com- 
plex, as  it  is  the  most  ^  aluable.  of  the  visual  faculties.  In  the 
performance  of  it,  the  visual  centre  goes  into  conference,  as  it 
were,  with  all  the  other  higher  centres  vi  the  brain,  and  asso- 
ciates the  image  with  all  the  other  experiences,  past,  present 
and  future,  that  have  any  sort  of  CDunection  with  the  (li)ject 
for  wliicli  thf  image  stands.  'I'his,  of  course,  is  the  ultimate 
end  of  vision,  which  enables  us  to  utilize  the  \isual  function 
for  the  i)urp(jses  of  li\iiig.  W  ithout  it.  the  value  of  \ision 
would  be  exceedingly  restricted.  We  would,  for  instance,  be 
able  to  see  and  recognize  an  autoin<tbile  in  the  street,  and  even 
to  calculate  \ery  accurately  its  distance  from  us,  but  we  should 
be  utterly  unable  to  axoid  it,  because  we  should  bi'  unable  to 
co-ordinate  all  this  inforniation  w  ith  the  other  ideas  necessary 


PHYSIOLOGY  OF  VISION  345 

-US""  'iP 
to  get  out  of  its  way.    Or  we  might  be  able  to  see  a  beautiful 

picture,  or  sunset,  and  even  to  enjoy  a  pleasurable  sensation  of 
its  beauty;  but  it  could  not  awaken  in  our  minds  all  the  train 
of  associated  ideas  which  would  move  us  to  joy  or  to  tears. 
So,  both  from  the  utilitarian  and  from  the  esthetic  standpoint, 
visual  association  is  the  most  valuable  of  all  the  higher  visual 
faculties  of  the  brain. 

It  is  highly  probable  that  not  one  of  these  higher  visual 
faculties  is  performed  by  the  centre  of  mind  vision  itself. 
Most  likely,  the  centre  in  the  frontal  convolution  merely  serves 
as  a  point  of  registration,  which  is  on  a  telegraphic  circuit,  so 
to  speak,  with  all  the  other  brain  centres  (through  the  associa- 
tion and  commissural  neurons)  and  the  entire  brain  takes  part 
in  the  performance  of  the  visual  faculties.  This  is  likely  to  be 
the  case  because  precisely  the  same  series  of  faculties  have  to 
be  exercised  by  the  brain  in  regard  to  every  one  of  the  senses 
in  order  to  make  them  of  any  working  value  to  the  individual. 
It  is  probable,  therefore,  that  the  cerebrum  is  a  clearing  house, 
to  which  all  the  brain  centres  refer  their  impressions  for  their 
appropriate  interpretation. 

VISUAL  JUDGMENTS. 

A  general  account  of  physiologic  judgments  has  already  been 
given  in  the  section  on  general  nervous  physiology,  and  a  brief 
description  of  visual  judgments  in  the  section  on  mind  vision. 
We  must  now  consider  these  latter  in  a  little  more  detail. 
They  are  so  closely  bound  up  with  the  function  of  vision  as  to 
be  almost  an  integral  part  of  the  sense  itself. 

There  are  three  important  conclusions  which  the  mind  has 
to  form  concerning  the  objects  which  it  views,  and  which  the 
sheer  perception  of  the  image  itself  does  not  afiford.  In  order 
to  arrive  at  these  conclusions  the  brain  is  obliged  to  utilize 
the  information  afforded  it  by  other  senses,  and  from  other 
sources. 

(1)  Judgment  of  Distance.  From  a  utilitarian  point  of  view 
this  is  perhaps  the  most  important  of  the  visual  judgments, 
since  it  enables  us  to  adjust  ourselves  in  our  active  relations 
with  objects  around  us — to  go  to  them,  to  avoid  them,  to  reach 
out  and  handle  them,  to  arrange  them,   etc.     Three  factors 


346  PHYSIOLOGY  OF  VISION 

enter  into  the  formation  of  the  judgment  of  distance:  muscle 
sense,  the  visual  angle,  and  experience.  The  first  pertains  to 
accommodation  and  convergence.  The  nearer  an  object  is  to 
us,  the  more  muscle  contraction  \vc  have  to  exert  in  the  ciliary 
and  rectus  muscles  in  order  to  see  it  clearly  and  singly.  By 
the  sense  of  this  muscle  contraction  we  form  a  judgment  of 
its  distance.  The  second  has  to  do  with  the  apparent  dimen- 
sion of  the  ol)ject  from  one  extreme  boundary  to  the  other, 
subtending  the  visual  angle.  This  we  compare  with  other 
objects  whose  distance  we  know  and  form  a  conclusion  as  to 
'  its  probable  distance.  The  third  factor  comprises  all  that  we 
have  previously  experienced  which  may  help  us  in  arriving  at 
a  conclusion.  This  last  factor,  of  course,  operates  also  in  the 
case  of  the  first  two. 

Of  the  three  factors,  the  muscle  sense  is  the  most  accurate 
and  reliable ;  but,  of  course,  its  operation  is  restricted  to 
objects  that  are  within  a  person's  far  point.  Beyond  his  far 
point,  he  does  not  use  accommodation  or  convergence,  and 
therefore  has  no  muscle  sense  to  go  by.  The  result  is  that 
inside  of  one's  far  point  judgments  of  distance  are  very  accu- 
rate ;  beyond  it,  they  are,  at  best,  only  vague  and  approximate. 
Everyone  knows  that,  as  he  looks  down  a  fairway  of  any  kind, 
every  point  and  object  within  about  20  feet  he  can  quite  def- 
initely locate,  and  could  easily  touch  them  if  he  had  a  pole 
long  enough;  beyond  that  distance,  he  could  only  touch  sume- 
where  near  them. 

The  second  factor,  the  visual  angle,  has  also  a  limited  use- 
fulness. In  order  to  judge  by  it  at  all,  there  must  be  several 
objects  of  known  size  and  distance,  witli  which  one  is  familiar, 
at  hand  for  comparison.  How  important  this  is  will  very 
(piickly  appear  when  one  tries  to  judge  distance  or  size  without 
such  familiar  objects  available.  A  familiar  example  is  to  be 
found  in  the  difference  in  the  apparent  si/e  and  distance  of  the 
moon  when  it  is  just  abo\e  the  horizon,  and  when  it  is  high  in 
the  heavens.  The  exi)lanalion  is  that  when  it  is  low,  we  view 
it  in  association  with  familiar  objects — trees,  houses,  churches, 
etc. — whose  size  and  distance  we  know,  and,  knowing  the  vast 
distance  of  the  moon  as  compared  with  llieir's,  we  imagine  it 


PHYSIOLOGY  OF  VISION  347 

looks  very  large.  When  it  is  in  the  zenith  we  view  it  all  by 
itself,  with  nothing  but  our  memory  of  things  to  compare  it 
with,  and  it  appears  smaller. 

When  the  distance  in  question  is  very  great,  i.  e.,  when  the 
visual  angle  is  very  small,  its  ^•alue  in  forming  judgment  of 
distance  is  nil.  We  cannot  form  judgments  of  objects  a  mile 
or  two  away  except  by  pure  reasoning  from  experience. 

At  long  distances,  therefore,  memory  and  experience  arc 
our  only  available  factors  for  forming  distance  judgments; 
and  such  judgments  are  exceedingly  vague  and  inaccurate. 

Still  another  factor  commonly  utilized  in  every-day  life  for 
judging  distance  is  the  extent  of  territory  and  the  number  of 
familiar  objects  which  lie  between  us  and  the  object  viewed. 
This,  however,  is  hardly  a  visual  judgment.  It  is  a  process  of 
reasoning. 

(2)  Judgment  of  Size.  Strictly  speaking,  there  is  no  such 
thing  as  a  judgment  of  size  in  the  visual  function,  because,  so 
far  as  sheer  vision  is  concerned,  there  is  no  such  thing  as  the 
size  of  an  object,  but  only  the  size  of  an  image  ;  and  that  needs 
no  judgment.  What  we  really  do  when  we  make  a  judgment 
of  size  is  to  judge  of  the  relative  size  of  a  certain  image  as 
compared  with  the  image  which  an  object  of  known  dimensions 
would  make  if  it  were  placed  at  the  same  distance  from  our 
eye,  a  yardstick,  for  example.  Judgment  of  size,  therefore,  is 
really  a  judgment  of  distance,  coupled  with  memory  and  ex- 
perience of  another  object.  Size  is  not  a  primary  quality  of 
an  object;  it  is  a  phase  of  distance. 

(3)  Judgment  of  Solidity,  or  the  Third  Dimension.  In 
judging  of  solidity,  the  brain  makes  use  of  two  factors,  dis- 
tance and  the  binocular  visual  field.  If  an  object  be  solid,  its 
projected  parts  will  be  further  from  our  eyes  than  its  frontal 
parts,  as  we  shall  conclude  from  the  use  of  our  accommodation 
and  convergence,  if  it  be  within  our  far  point,  and  by  the  dif- 
ference of  visual  angle  (what  the  artist  calls  perspective). 
Further,  since  the  outer  parts  of  the  image  fall  on  different 
areas  of  our  two  retinae  (see  "\'isual  Field")  and  some  parts 
of  it  do  not  fall  on  both  retinae  but  only  on  one  or  the  other, 
the  images  which  we  get  of  a  solid  object  on  the  right  and 


348  PHYSIOLOGY  OF  VISION 

left  retinae  respectively  will  he  (juite  different.  (This  is  easily 
proved  by  shutting  first  one  eye  and  then  the  other  in  view- 
ing a  solid  object.)  By  both  of  these  factors,  when  the  object 
is  within  our  far  point,  and  by  the  latter  alone  when  it  is 
beyond  infinity,  we  form  a  judgment  of  solidity.  At  very 
long  distances  both  factors  fail  us,  and  judgment  of  solidity  is 
then  impossible.  We  then  only  know  that  an  object  is  solid 
because  we  recognize  it  as  a  familiar  object,  this  recognition 
being  based  on  previous  experience. 

In  common  with  all  physiologic  judgments,  visual  judg- 
ments never  err;  and  for  this  reason  they  are  readily  made  the 
media  of  apparent  deception.  Upon  this  fact  are  based  all  the 
illusions  of  the  artist's  pencil,  the  painter's  brush,  and  the  stage, 
and  anyone  with  ingenuity  can  work  out  for  himself  a  hundred 
amusing  devices  to  deceive  the  brain  through  the  visual  judg- 
ments. One  of  the  most  familiar  of  such  deceptions  is  the 
stereoscope,  which  deceives  the  brain  into  thinking  that  a 
plane  picture  has  solidity  by  presenting  to  each  retina  a  dif- 
ferent view  of  the  scene,  corresponding  to  the  different  views 
which  the  solid  objects  themselves  would  present,  and  by 
means  of  a  pair  of  prisms,  base  in,  compelling  the  viewer  to 
use  convergence  of  a  different  degree  for  \arious  parts  of  the 
picture. 

COLOR  PERCEPTION. 

The  perception  of  color  is  an  integral  part  of  the  sense  of 
vision  ;  that  is  to  say,  it  cannot  be  sei)arated  from  the  perform- 
ance of  the  act  of  vision,  and  the  sensation  of  color  cannot  be 
separated  from  the  sensation  of  light. 

The  subjective,  or  physiological,  aspects  of  color  are  not  so 
well  understood  as  its  ol)jecti\c,  or  physical,  side.  It  is  gen- 
erally supposed  that  in  the  \isual  purple  there  are  different 
kinds  of  jjhoto-chemical  substance,  each  of  which  reacts  to  a 
different  wave-length,  thus  ])r()<Iiicing  the  sensation  in  the  brain 
of  a  different  color.  As  to  tlu-  dct.iils  of  this  process,  tlitie 
are  many  different  llu-oiii-s  ;i(l\;uici'(l  bv  various  scientists. 

The  Young-llelmhol/  theory  holds  tli:it  there  are  three  such 
substances,  reacting  to  wa\e-lenj;ths  which  i)roduce  the  sensa- 
tions of   ri'd,   green   and    blue,   respectix  ely.      It'   either   one   of 


PHYSIOLOGY  OF  VISION  349 

these  three  substances  be  stimulated  alone,  it  produces  the 
single  sensation  of  red,  or  green,  or  blue,  as  the  case  may  be. 
If  two  are  stimulated  at  the  same  time,  a  sensation  is  produced 
of  a  mixed  color — e.  g.,  if  red  and  blue  are  stimulated  together 
the  resulting  sensation  is  purple.  In  fact,  all  other  color  sen- 
sations are  produced  by  mixed  stimulation  of  two  or  more  of 
the  substances,  in  varying  degrees  of  intensity,  just  like  the 
mixing  of  paints  together  in  varying  proportions.  Equal 
stimulation  of  all  three  gives  the  sensation  of  white  light. 

Another  theory,  the  Hering  theory,  also  assumes  three  dif- 
ferent photo-chemical  substances,  but  ascribes  to  each  a  two- 
fold action,  namely,  a  decomposition  or  tearing-down  under 
the  influence  of  certain  wave-lengths,  and  a  synthesis  or  build- 
up under  the  influence  of  certain  other  wave-lengths ;  known 
as  catabolic  and  anabolic  actions,  respectively.  Both  the  tear- 
ing-down and  the  building-up  of  each  of  the  three  substances 
give  different  color  sensations.  The  double  sensations  thus 
ascribed  to  the  three  swbstances  are  as  follows :  White-Black  ; 
Red-Green;  Yellow-Blue. 

Other  color  sensations,  under  the  Hering,  as  under  the 
Young-Helmholz  theory,  are  due  to  mixed  stimulations. 

The  three  color  sensations  produced  by  single  stimulation 
under  the  Young-Helmholz  theory,  and  the  six  predicated  by 
Hering,  are  known  as  primary  colors.     (See  Color.) 

COMPLEMENTARY   COLORS. 

If  the  eye  gazes  for  several  moments  steadily  upon  a  single, 
or  primary  color,  and  then  it  be  either  closed  or  else  fixed 
upon  an  area  of  neutral  tint,  an  area  of  another  color  w\\\  be 
seen,  similar  in  size  and  shape  to  that  of  the  primary  color 
originally  gazed  upon.  Thus,  if  one  first  look  at  an  area  of 
red.  the  after-color  will  be  green  :  if  blue,  then  the  after-color 
is  yellow.  These  colors  are  said  to  be  complementary  of  each 
other. 

Young  and  Helmholz  explain  complementary,  or  rather 
after-colors,  on  the  ground  that  after  gazing  for  a  while  at  the 
primary  color  the  photo-chemical  substances  in  the  retina  cor- 
responding to  that  color  become  exhausted  and  unable  to 
function  :  so  that  when  the  gaze  is  transferred  to  a  white  or 


350  PHYSIOLOGY  OF  VISION 

neutral  area,  where  all  the  three  color-waves  are  stimulating 
the  retina,  only  the  other  two  substances  respond,  and  the  re- 
sulting sensation  is  a  mixture  of  the  other  two  primary  colors. 
Thus,  if  w^e  look  steadih-  at  red,  the  red  sul)stance  is  exhausted, 
and  when  we  turn  to  a  neutral  area  (jnly  the  green  and  blue 
substances  respond,  giving  the  sensation  of  a  bluish  green. 
When  blue  is  gazed  at  first,  then  the  red  and  green  form  the 
after-color,  gixing  a  sensation  of  reddish  yellow.  It  was  a 
study  of  complementary  colors  that  led  Young  and  Helmholz 
to  formulate  their  theory  of  color  sensations. 

Hering  explains  after-colors  on  the  ground  that  they  repre- 
sent the  tearing  down  and  the  building  up  of  the  same  photo- 
chemical substances,  which,  of  course,  naturally  follow  each 
other.  So,  if  we  look  steadily  at  red.  our  red-green  substance 
is  being  torn  down  :  turning  to  a  neutral  area,  this  substance 
now  begins  to  build  itself  up,  and  in  so  doing  gives  us  the  sen- 
sation of  green.  If  we  first  look  at  green,  the  substance  is  be- 
ing built  up,  and  when  we  turn  to  a  neutral  area  it  begins  to 
be  torn  dow^n.  giving  the  sensation  of  red.  .\nd  so  with  all 
the  photo-chemical  substances. 

It  may  be  said,  in  passing,  that  there  arc  many  facts  and 
phenomena  in  regard  to  color  which  cannot  be  explained  on 
either  of  these  two  theories,  and  some  which  seem  to  be  incon- 
sistent with  both  of  them.  We  must,  therefore,  conclude  that 
no  satisfactory  theory  has  yet  been  formulated  to  account  fully 
for  all  the  phenomena.  The  field  presents  opportunities  for  in- 
teresting investigation. 

COLOR   BLINDNESS. 

Some  persons  are  physi(;Iogically  incapable  of  distinguishing 
certain  colors;  a  few.  indeed,  are  unable  to  distinguish  color 
at  all  and  live  all  their  li\es  in  a  world  of  black-and-white, 
but  they  are  very  rare.  Such  a  coiulition  is  known  as  color 
blindness,  partial  or  com])lete.  It  is  generally  sui)posr(l  to  be 
due  to  the  lack  in  the  retina  of  those  chemical  substances  which 
react  to  the  stimulation  of  the  wa\ c-lengths  of  the  coktr.  or 
colors,  to  which  tlu-y  an-  blind. 

.According  to  the  theories  of  both  \'oung-l  Iilmlu»l/  .'ind 
liering,  we  should   naturally  expect  to  find   ;i   person   who  is 


PHYSIOLOGY  OF  VISION  351 

blind  to  one  of  the  primary  colors  also  blind  to  its  comple- 
mentary color;  and,  in  general,  this  is  what  we  find  in  actual 
practice.  The  commonest  color-blindness  is  for  red  and  green. 
However,  there  are  here  and  there  persons  met  with  who  ap- 
pear to  be  blind  to  one  of  these  two  colors  and  not  to  the 
other — which  is  rather  upsetting  to  both  theories.  It  is  one  of 
the  things  not  yet  satisfactorily  accounted  for. 

IN  INDUSTRIAL  PURSUITS. 

In  certain  industrial  pursuits,  notably  in  railroad  work, 
where  colors  are  used  for  signalling,  the  detection  of  color 
blindness  is  highly  important,  to  avoid  accidents.  Unfor- 
tunately, red  and  green,  to  which  most  color-blind  persons  are 
blind,  are  the  two  colors  most  used  for  signal  purposes,  be- 
cause they  carry  the  farthest  and  through  the  worst  weather, 
and  also  because,  being  complementary  colors,  they  are  most 
easily  distinguished  from  each   other. 

The  color-blind  person  sees  mixed  colors  without  the  ad- 
mixture of  the  particular  color  to  which  he  is  blind.  Thus,  if 
we  show  a  person  who  is  red-blind  a  purple  color,  which  is  a 
mixture  of  red  and  blue,  he  does  not  see  the  red  in  it,  but  only 
the  blue ;  to  him  it  appears  blue.  If  we  show  him  an  orange, 
which  is  a  mixture  of  red  and  yellow,  he  sees  only  the  yellow. 
If  we  show  him  a  mixture  of  many  colors,  containing  his  blind- 
color,  but  none  of  them  in  great  saturation,  he  sees  them  as  a 
greyish  hue.  These  are  called  his  confusion  colors;  and  we 
make  use  of  them  to  test  his  color  sense  or  blindness.  All  tests 
for  color  blindness  are  based  on  this  principle. 

Different  colored  surfaces,  when  polished,  have  character- 
istic glares,  which  a  color-blind  person  is  sometimes  able  to 
differentiate  from  each  other,  and  thus  to  pick  out  colors  to 
which  he  is  blind.  To  obviate  this,  Holmgren  devised  the  plan 
of  making  the  test  with  colored  skeins  of  wool,  upon  which 
there  is  no  glare,  but  a  dead,  saturated  color.  Eldridge-Green, 
of  London,  has  devised  a  set  of  tests  with  lanterns,  on  the 
ground  that  the  industrial  use  of  colors  is  usually  in  the  form 
of  transmitted  lights ;  and  this  test  has  now  become  fairly 
standard  with  English  railroads.  This  scientist  has  also  worked 
out  a  distinctive  theory  of  color  perception  in  general,  which, 
however,  is  too  extensive  to  go  into  here. 


352  PIGMENT 

Pigment.     Coloring  matter  in  the  tissues. 

Pigmentum  Nigrum.    The  l)lack'  pigment  in  the  choroid. 

Pile  Reflex.  Contraction  of  the  pupil  on  having  the  attention 
suddenly  called  to  an  object. 

Pilocarpine,  An  alkaloid  of  jaborandi,  used  in  the  eye  as  a 
myotic,  to  contract  the  pupil  and  reduce  intra-ocular  pressure. 
It  is  not  quite  so  powerful  or  so  irritating  as  eserine,  and  is 
therefore  often  used  in  place  of  that  drug.  It  is  employed  in 
aqueous  solution  of  from  one-half  to  four  per  cent. 

Pince-Nez.    A  term  for  nose-glasses. 

Pinguecula.  A  small  yellowish  elevation  at  the  limbus.  seen 
in  old  people. 

Pin-Hole  Disc.  An  ojjaque  disc,  with  a  pinhole  in  the  center. 
When  mounted  before  the  eye,  this  disc  cuts  out  all  but  the 
central  rays  of  light,  which,  traveling  along  or  near  the  prin- 
cipal axis,  are  practically  uninfluenced  by  the  refraction  of  the 
eye.  Where  the  eye  has  poor  vision,  therefore,  this  disc  fur- 
nishes a  test  as  to  whether  the  trouble  is  due  to  refractive 
error  or  to  disease.  If  to  the  former,  the  pin-hole,  by  eliminat- 
ing the  peripheral,  refracted  rays,  does  away  with  the  eti'ects 
of  the  error,  and  makes  vision  better.  If  to  the  latter,  it  does 
not  impro\e.  but  rather  makes  vision  worse,  by  shutting  out 
light   from  a  disabled  retina.     The  general  rule  is  that  if  the 


I'jll-llcl..      1  llh.   . 


PIN-HOLE  IMAGE  353 

pinhole  disc  improves  vision,  a  lens  can  be  found  which  will 
improve  it.  The  rule  must  not,  however,  be  taken  too  abso- 
lutely. 

Pin-Hole  Image.  An  image  made  on  a  screen  by  means  of  inter- 
posing an  opaque  screen  with  a  pin-hole  in  it  between  the 
object  and  the  image-screen.  There  are  a  certain  number  of 
rays  from  every  point  on  the  central  portion  of  the  object 
which  pass  as  principal  rays  through  the  pin-hole,  and,  falling 
on  the  screen,  define  an  inverted  image.  Naturally,  it  is  neither 
a  very  clear  image  nor  a  very  intense  one,  since  it  represents 
only  a  small  proportion  of  light  from  the  object.  It  becomes 
less  intense  as  the  receiving-screen  is  moved  further  away 
from  the  pin-hole,  but  its  clarity  is  the  same  wherever  the 
screen  is  held.  It  can  be  made  better-defined  by  refocalizing 
through  a  convex  lens. 

Pin-Hole  Test.  If  an  opaque  disc  with  a  pin  hole  in  the  center 
be  placed  before  the  eye,  so  that  the  pin  hole  coincide  with 
the  center  of  the  pupil,  then  the  only  light  which  is  admitted 
to  the  eye  is  that  which  passes  along  the  principal  axis,  and 
approximately  thereto.  As  this  light  undergoes  practicallv 
no  refraction,  the  images  made  by  it  on  the  retina  are  not 
affected  by  any  error  of  refraction  the  subject  may  have; 
hence,  in  a  patient  whose  vision  is  ordinarily  affected  by  an 
error  of  refraction,  the  pin-hole  disc  will  sharpen  and  clarify 
the  vision.  On  the  other  hand,  in  a  patient  whose  vision  is 
aff'ected  because  of  a  retinal  disease,  the  pin  hole  will  make 
vision  still  worse,  because  the  central  part  of  the  retina  suf- 
fers most  Irorn  organic  disease. 

Pink  Eye.  A  highly  contagious  form  of  catarrhal  conjunctivitis, 
in  which  the  eyeball  is  of  a  pink  color. 

Pladarosis.     A  soft  tumor  on  the  eyelid. 

Plane.     A  surface  such  that  if  any  two  points  in  it  are  joined 

by  a  straight  line,  that  line  will  lie  wholly  within  the  surface. 

As  applied  to  mirrors  and  lenses,  the  term  means  that  the 

surface  of  the  mirror  or  the  lens  lies  in  a  single  plane,  i.  e., 

that  it  has  no  curvature  whatever. 


354  PLANE,   PRINCIPAL 

Plane,  Principal.  A  plane  dropped  from  the  principal  point  of 
a  lens  perpendicular  to  its  principal  axis. 

Piano-Concave.     Piano  on  one  side  and  concave  on  the  other. 

Piano-Convex.     Piano  on  one  side  and  convex  on  the  other. 

Plica.    A  fold  of  skin  or  mucous  membrane. 

Plica  iridis.     A  fold  on  the  posterior  surface  of  the  iris. 
Plica  semilunaris  conjunctivae.     A  fold  of  the  conjunct i\  a 
at  the  inner  canthus. 

Points.  In  every  lens  there  are  four  points,  lying  along  its  prin- 
cipal axis,  from  which,  being  knov^n,  all  the  mathematical 
problems  of  its  action  can  be  calculated.  These  are  known  as 
the  cardinal  points  of  the  lens,  and  are  as  follows : 

(1)  The  first,  or  anterior,  principal  focus,  i.  e.,  the  point 
anterior  to  the  lens,  light-waves  proceeding  from  which  are 
rendered  neutral  upon  entering  the  lens. 

(2)  The  second,  or  posterior,  principal  focus,  i.  e.,  the  point 
posterior  to  the  lens  at  which  neutral  waves  in  the  lens  are 
brought  to  a  focus  on  emerging  from  the  posterior  surface. 

(3)  The  first,  or  anterior,  principal  point,  i.  e.,  the  point 
from  which  to  the  first  principal  focus  the  focal  length  of  the 
first  refracting  surface  of  the  lens  is  measured. 

(4)  The  second,  or  posterior,  principal  point,  i.  e..  the 
])oint  from  which  to  the  second  principal  focus  the  focal 
length  of  the  i)Osterior  lens  surface  is  measured. 

As  a  matter  of  fact,  the  two  principal  points  can  no  more  be 
regarded  separately  llian  the  blades  of  a  pair  of  scissors.  They 
must  be  considered  and  defined  as  a  pair.  IMiey  are  the  points 
of  separation  of  the  focal  systems  of  the  two  surfaces  by  the 
thickness  of  the  lens. 

All  four  of  these  points  are  joint  functions  of  the  thickness 
of  the  lens,  its  index  of  refraction,  and  the  racbi  of  curvature 
of  its  two  surfaces. 

In  addition  to  the  four  canlinal  ])oints,  a  lens  has  two  nodal 
points, — points  within  the  lens  on  the  princii)al  axis  from  which 
the  object  and  the  image  appear  under  the  same  angle.  I'or 
fmthcr  description  of  these  points  see  Nodal  Points. 


POLE  355 

In  thin  ophthalmic  lenses  the  principal  and  nodal  points 
are  regarded  as  being  identical  with  the  optical  center  of  the 
lens. 

The  eye,  being  a  triple  compound  lens  system,  has  three 
sets  of  cardinal  and  nodal  points  situated  along  its  principal 
axis ;  but  for  working  convenience  three  systems  are  calculated 
as  one,  the  position  of  the  points  being  averaged  as  follows: 

The  principal  point,  where  the  imaginary  surface  of  the  re- 
duced eye  cuts  the  principal  axis,  2.34  mm.  behind  the  anterior 
surface  of  the  cornea. 

The  nodal  point,  .476  mm.  in  front  of  the  posterior  surface  of 
the  crystalline  lens. 

The  posterior  principal  focus,  (in  the  emmetropic  eye),  where 
the  retina  cuts  the  principal  axis. 

The  anterior  principal  focus,  varying  as  to  its  position  with 
the  dioptrism  of  the  eye. 

Point  of  Reversal.  The  point  where  a  minus  light  wave 
focusses  and  reverses  itself  into  a  plus  wave.  In  technique  this 
term  refers  to  the  focussing  and  reversing  of  the  emergent 
wave  from  the  patient's  eye  in  retinoscopy.    See  Retinoscopy. 

Pole.  Geometrically,  this  is  the  name  given  to  either  extremity 
of  the  diameter  or  axis  of  a  sphere.  In  physics,  a  pole  is  one  of 
the  points  of  a  body  at  which  the  attraction  or  repulsion  energy 
is  concentrated,  as  the  poles  of  a  magnetic  needle,  the  poles 
of  a  battery,  etc. 

In  optics  the  word  is  used  geometrically.  The  poles  of  a 
lens  are  the  points  where  the  two  surfaces  cut  the  principal 
axis,  i.  e.,  the  anterior  and  posterior  poles.  The  same  holds 
good  of  the  crystalline  lens  of  the  eye. 

Polarimeter.  An  instrument  for  measuring  the  rotation  of 
polarized  light.     See  Polarization. 

Polariscope.  An  optical  instrument  for  exhibiting  the  polariza- 
tion of  light,  or  for  examining  transparent  media  for  the  pur- 
pose of  determining  their  polarizing  power.     See  Polarization. 

Polarization.  A  wave  of  light,  on  entering  certain  crystalline 
media,  can  give  rise  to  two  refracted  pencils,  one  being  re- 


356  POLYCHROMATE 

fracted  according  to  the  ordinary  laws  of  refraction,  the  other 
according  to  a  law  determined  by  the  form  and  orientation  of 
the  surface  of  the  crystal  (Huy gens'  Law).  The  first  is  called 
the  ordinary  ray,  the  second  the  extraordinary  ray.  In  the 
case  of  Iceland  spar,  and  several  other  crystals,  the  extraor- 
dinary ray  is  refracted  away  from  the  axis,  but  in  many  cases 
this  is  reversed.  Crystals  of  the  former  kind  are  called  posi- 
tive, and  of  the  latter  kind  negative. 

AVaves  that  have  been  extraordinarily  refracted  acquire  cer- 
tain new  properties  with  respect  to  transmission  through  a 
second  crystal,  and  are  said  to  be  polarized.  In  addition  to  this 
process  of  double  refraction,  light  may  be  polarized  in  the  fol- 
lowing ways : 

Reflection  at  a  proper  angle  (the  polarizing  angle)  from  the 
surfaces  of  transparent  media,  as  glass,  water,  etc. 

Transmission  through  a  sufficient  number  of  transparent  un- 
crystallized  plates  at  proper  angles. 

Transmission  through  a  number  of  other  ]}odies  imperfectly 
crystallized,  as  agate,  mother-of-pearl,  etc. 

Diffraction  through  a  grating. 

Polarized  light  cannot  be  distinguished  from  ordinary  light 
by  the  naked  eye.  To  demonstrate  polarization  two  pieces  of 
apparatus  are  necessary,  one  to  polarize  the  light,  the  other  to 
show  the  polarization.  The  former  is  called  the  polarizer, 
the  latter  the  analyzer.  And  every  apparatus  w^hich  serves  one 
purpose  will  serve  the  other  also.  The  usual  process  consists 
in  viewing  the  examined  light  through  the  analyzer,  and  ob- 
serving whether  any  change  of  brightness  occurs  as  the 
analyzer  is  rotated.  There  are  two  positions,  differing  by  18C) 
degrees,  which  give  a  mininumi  of  brightness;  and  the  two 
positions  intermediate  between  these  gi\e  a  maximum.  The 
extent  of  the  changes  thus  observed  is  a  measure  of  the  com- 
pleteness of  polarization. 

The  practical  value  of  polarized  light  lo  tlu-  refr.ictioiiist  and 
other  workers  in  optics  is  that  it  is  easier  to  control  and  is 
not  reflected  in  a  thousand  dilTcreiit  planes,  as  ordinary  li,i;ht 
is.     (Sec  Light.) 

Polychromate.     <  )ne  who  is  al)le  to  (ii>tiiiguiNli  many  i-olorv,. 


POLYCORIA  357 

Polycoria.     More  than  one  pupil. 

Polyopia.     Multiple  vision. 

Pop-Eyed.    Having  protruding  eyeballs. 

Porus  Opticus.  The  opening  through  the  lamina  cribrosa  giving 
passage  to  the  central  retinal  artery  and  vein. 

Positive.  In  optics  the  term  refers  to  geometric  points  that 
are  actual,  and  their  measurement  and  relationships.  Thus, 
positive  foci  are  points  where  light  waves  are  actually  brought 
to  a  focus. 

The  word  is  also  loosely  used  to  indicate  expanding  light 
waves,  and  those  reflecting  and  refracting  surfaces  which 
produce  them.  Such  waves  and  surfaces  are  better  termed 
plus. 

Post-Ocular.    Situated  behind  the  eyeball. 

Post-Ocular  Neuritis.  Inflammation  of  that  part  of  the  optic, 
nerve  posterior  to  the  eyeball.  It  is  more  commonly  called 
Retrobulbar  Neuritis. 

Postopticus.  Any  one  of  the  nerve  centers  or  sub-centers  back 
of  the  optic  nerve. 

Prelachrymal.     In  front  of  the  lachrymal  sac. 

Presbyopia.  Literally,  the  sight  of  old  age.  The  term  is  used 
to  denote  that  physiologic  state  in  which,  by  reason  of  the 
hardening  of  the  crystalline  lens,  the  individual  is  no  longer 
able  to  accommodate  for  his  convenient  near  point.  To  this 
pass  every  emmetropic  and  hyperopic  person,  and  every  myope 
whose  myopia  is  less  than  3  D.,  comes  at  Lome  time  about 
middle  age ;  and,  as  a  matter  of  fact,  high  myopes  come  to  the 
same  pass,  but,  owing  to  their  static  refractive  conditions,  they 
need  no  accommodation  for  near  point,  but  quite  the  reverse. 

In  former  days  the  commencement  of  presbyopia  was  arbi- 
trarily fixed  at  the  recession  of  the  near  point  to  33  cm.  The 
process  of  lens-hardening,  of  course,  begins  quite  early  in  life, 
and  steadily  progresses  with  the  years ;  and  with  this  gradual 
loss  of  accommodation  the  near  point  gradually  recedes  further 


358  PRIMARY 

and  further  from  the  eye.  When  it  reached  ?>3  cm.,  which 
means  that  th<i  accommodation  dropped  to  3  D.,  presbyopia 
was  said  to  have  begun.  This  point  was  geneially  supposed 
to  be  reached  at  45  years  of  age ;  hence  our  tables  formerly 
showed  45  years  as  the  age,  and  3  D.  as  the  dioptric  ampli- 
tude, at  which  presbyopia  commenced. 

Nowadays,  however,  we  do  not  attempt  to  fi.x  either  the  age 
or  the  amplitude  at  which  presbyopia  begins.  A  person  be- 
comes presbyopic,  as  stated  at  the  outset,  when  his  amplitude 
of  accommodation  is  no  longer  adequate  to  negotiate  the  near 
point  at  which  he  wishes  to  read,  or  to  do  his  customary  work. 
The  age  at  which  people  lose  their  accommodative  power 
varies  greatly ;  so  does  the  accommodative  demand  in  different 
individuals.  Further,  we  now  know  that  a  certain  amount  of 
accommodative  reserve  is  necessary — or  at  least  desirable — 
for  comfortable  near  vision;  so  that  the  exact  dioptric  value 
of  the  amplitude  is  not  a  correct  measure  of  presbyopia ;  and 
this  reserve  dififers,  also,  m  different  persons. 

The  clinical  problems  of  presbyopia  are  essentially  those  of 
insufficient  accommodation.  ITnder  our  present  knowledge  of 
the  subject  it  is  no  longer  permissible  to  regard,  or  to  treat, 
presbyopia  as  an  entity ;  to  merely  find  the  point  at  which 
the  patient  reads,  divide  the  distance  into  unity,  and  add  to 
the  amplitude  thus  found  the  dioptres  of  lens  power  which 
will  bring  it  up  to  the  proper  figure.  It  is  really  better  that 
the  term  "])rcsbyopia"  should  be  left  out  of  mind  altogether, 
and  each  individual  case  thoroughly  investigated  and  handled 
as  a  problem  in  accommodation  and  in  accommodation-con- 
vergence relation.  For  details  of  such  investigation  and  treat- 
ment the  reader  is  referred  to  the  sections  on  Accommodation 
and  Convergence. 

Primary.  .As  a])plied  to  two  or  more  plienomena.  or  problems, 
or  functions,  it  refers  to  the  thing-in-chief,  ol  which  the  other 
or  others  arc  derivatives. 

Primary  Colors.  Colors  produced  by  singii-  stimulation  of 
the  photo-chemical  substances  in  the  letina. 

Primary  l)e\iation.  'J'hc  de\iation  made  by  ;i  S(|uintin^  eye 
when  the  scjund  eye  is  fixing. 


PRINCIPAL   FOCUS  359 

Primary  Position.  The  position  of  the  eyes  in  convergence 
from  which  all  others  are  calculated,  i.  e.,  position  of  rest. 

Principal  Focus.  The  point  on  the  principal  axis  where  a  spheri- 
cal mirror  or  lens  brings  neutral  light  waves  to  a  focus. 

Principal  Meridians.  The  meridians  of  greatest  and  least  curva- 
ture in  an  astigmatic  eye.     (See  Astigmatism.) 

Principal  Points.     See  Points. 

Prism.  A  prism  may  be  defined  as  a  homogeneous  refracting 
body  having  two  plane  refracting  surfaces  inclined  toward 
each  other.  In  ophthalmic  practice  prisms  are  usually  made  of 
crow  n  glass,  the  same  as  ophthalmic  lenses.  In  shape,  i.  e.,  in 
the  arrangement  of  its  refracting  and  non-refracting  surfaces, 
a  prism  may  be  rectangular,  triangular,  or  circular.  Rectangu- 
lar prisms  are  used  only  in  experimental  optics ;  for  clinical 
purposes  we  employ  almost  exclusively  the  circular  form,  be- 
cause it  fits  conveniently  into  the  trial  frame. 

The  place  where  the  two  refracting  surfaces  meet  each  other 
is  called  the  apex  of  the  prism ;  the  place  where  the  distance 
between  these  surfaces  is  greatest,  the  base;  a  perpendicular 
dropped  from  the  apex  to  the  base  is  called  the  apex-base  line. 

In  conformity  with  the  laws  of  refraction,  a  wave  of  light 
which  falls  upon  the  surface  of  a  prism  other  than  perpendicu- 
larly to  that  surface  (entering  ray)  is  refracted  toward  the  per- 
pendicular of  the  surface,  i.  e.,  toward  the  base.  It  travels  un- 
changed through  the  prism,  and  upon  emerging  at  the  other 
surface — provided  it  emerges  other  than  perpendicularly — is 
refracted  away  from  the  perpendicular,  i.  e.,  once  again  toward 
the  base  (emerging  ray).  The  net  effect  of  a  prism,  therefore, 
upon  the  refracted  wave,  or  ray,  is  to  bend  it  around  its  base. 

MAXIMUM  AND  MINIMUM  POWER. 

The  degree  of  bending  power  thus  exercised  by  a  prism  varies 
according  to  the  angle  at  which  the  wave  enters  or  emerges 
from  it.  Its  greatest  power  is  exercised  when  all  the  bending 
is  done  at  one  surface,  i.  e.,  when  the  ray  either  enters  or 
emerges  perpendicular  to  the  surface.  The  ray  is  bent  the 
least  when,  upon  entering,  it  is  made  to  pass  through  the  prism 


360  PRISM 

parallel  with  the  base.  These  constitute,  respectively,  the 
maximum  and  minimum  bending  power  of  the  prism.  Its 
middle,  or  medium  power  is  exercised  when  the  ray  enters  the 
l)rism  parallel  with  the  base.  It  is  this  medium  power  whic^ 
we  generally  invoke  in  clinical  practice. 

DEVIATION. 

Everything  else  being  equal,  the  degree  of  deviation  which 
a  prism  gives  to  a  ray  of  light  depends  upon  the  angle  which 
the  two  refracting  surfaces  make  with  each  other;  that  is  to 
say,  the  apex  angle;  and  this  angle  is  therefore  known  as  the 
angle  of  refraction.  Originally  prisms  were  designated  by 
this  angle. 

Later,  it  was  demonstrated  that  the  average  deviation  of  a 
prism,  that  is  to  say,  the  deviation  it  gives  to  a  ray  which  en- 
ters parallel  to  the  base-line,  is  equal  to  about  half  the  apex 
angle,  and  this  half  angle,  known  as  the  prism  angle,  because 
the  unit  of  designation  for  prisms.  Neither  of  these  two  meth- 
ods of  designation  was  very  satisfactory,  however,  because 
they  represented  the  physical  properties  of  the  prism  rather 
than  its  actual  deviating  power, 

Dennet  was  the  first  to  devise  a  system  of  numeration  for 
prisms  in  terms  of  the  deviation  which  it  actually  gave  to  a 
ray  of  light.  He  measured  the  deviation  l)y  the  arc  subtending 
the  angle  of  deviation,  taking  as  his  unit  the  one-hundredth 
part  of  the  radian,  which  he  called  a  centrad. 

Still  later.  Prentice  proposed  a  method  of  measuring  the 
deviation  by  means  of  a  tangent  to  the  arc,  perpendicular  to 
the  normal  ray,  from  the  point  where  this  tangent  cuts  the 
normal  ray  to  the  i)oint  where  it  cuts  the  deviateil  ray.  Pren- 
tice took  for  his  unit  the  tangent  of  de\  iation  thus  shown  at  a 
distance  of  1  meter  from  the  entering  surface  of  the  prism, 
when  that  deviation  measured  1  centimeter.  'Jhis  unit  of 
deviation  he  called  a  prism  dioptre.  This  system  of  luiincra- 
tion,  which  is  now  almost  universally  einplovftl  in  this  coun- 
try, has  three  decided  advantages: 

(1)  It  aflfords  a  ready  formula  for  llu-  calculation  of  tlexial- 
ing  power  at  any  distance  by  means  of  a  plane  screen  or  chart. 
If  I)  stands  for  the  distance  at  which  the  mcasuroincnt  is  made. 


PRISM  361 

d  for  the  amount  of  deviation  in  centimeters,  and  p.  d.  for  the 
prism  dioptres,  then : 

d 
p.  d.  =  - 

D 

(2)  It  gives  us  a  system  of  mensuration  and  calculation 
which  coincides  with  the  dioptric  system  of  designating  lenses 
— a  feature  of  great  practical  value  in  decentration  problems. 

(3)  It  permits  of  the  direct  practical  application  of  prism 
measurement  to  the  diagnosis  and  correction  of  errors  of  con- 
vergence, with  their  relations  to  accommodation. 

APPARENT  DISPLACEMENT  OF  IMAGE. 

In  viewing  an  object  through  a  prism,  the  rays  of  light 
which  reach  the  eye  from  the  object  are  bent  around  the  base 
of  the  prism;  the  eye,  however,  projects  the  rays  back  along 
a  straight  line  continuous  with  the  line  which  the  emergent 
rays  traversed  from  the  prism  to  the  eye ;  there  is,  therefore, 
always  an  apparent  displacement  of  the  image  toward  the  apex 
of  the  prism. 

PRISMATIC   ABERRATION. 

Not  only  is  the  image  of  an  object  apparently  displaced  by 
viewing  it  through  a  prism,  but  it  is  distorted  as  well.  This 
is  due  to  what  is  known  as  prismatic  aberration.  In  addition 
to  the  ordinary  forms  of  aberration  which  the  prism  shares 
with  all  refracting  bodies,  it  possesses  a  peculiar  aberration  of 
its  own,  due  to  the  fact  that  no  constant  ratio  exists  between 
the  angle  of  incidence  and  the  angle  of  refraction,  but  only  be- 
tween the  sines  of  those  angles.  All  homocentric  light,  i.  e., 
all  light  proceeding  from  or  going  toward  a  point,  after  travers- 
ing a  prism  is  no  longer  homocentric.  This  brings  about  a 
lateral  aberration,  or  astigmatism,  which,  in  the  wearing  of 
high-power  prisms,  gives  a  great  deal  of  trouble. 

v 

DISPERSION. 

J\.\\  prisms  possess  the  property  of  breaking  light  up  into 
its  component  color  waves,  since  the  different  color-waves  are 
unequally  refrangible,  and  are  deflected  through  different 
angles.  This  phenomenon  is  known  as  dispersion.  The  violet 
rays  are  deflected  the  least,  and  red  the  most,  and  the  inter- 


362  PRISM 

mediate  colors  are  spread  out  between  these  two  extremes, 
forming  the  spectrum.  This  property  of  dispersion  is  of  great 
^•alue  in  spectrum  analysis,  but  a  serious  disadvantage  in  the 
ophthalmic  use  of  prisms  because  of  the  colored  margin  it  gives 
to  every  object  in  proportion  to  the  strength  of  the  prism  used. 
Prisms  for  clinical  use  are  made  of  crown  glass  because  its 
dispersive  power  is  much  less  than  that  of  flint  glass,  being  as 
33  to  52.  Achromatic  prisms  can  be  made  by  combining  these 
two  kinds  of  glass  in  proper  relationship. 

ROTATING  PRISMS. 

Sir  John  Herschell  first  demonstrated  that  l)y  placing  two 
prisms  in  apposition  and  rotating  them  in  opposite  directions 
it  was  possible  to  produce  the  effect  of  a  single  increasing  or 
decreasing  prism.  If  two  prisms  of  equal  strength  be  placed, 
apex  to  base,  they  neutralize  each  other,  so  that  the  prism 
effect  is  zero.  The  further  they  are  rotated  from  this  position 
the  greater  becomes  the  prism  effect  of  the  combination,  until, 
when  they  lie  apex  to  apex,  the  strength  of  the  combination 
becomes  equal  to  the  sum  of  the  two  prisms. 

This  principle  underlies  the  various  rotary  prisms  anil 
phorometers  which  are  on  the  market,  for  use  in  the  testing 
and  exercising  of  the  ocular  muscles. 

CLINICAL  USE  OF  PRISMS. 
Since  a  prism  can  deflect  the  path  of  a  light-wa\e  to  any 
given  degree,  depending  upon  the  size  of  its  apex  angle,  it  is 
evident  that  it  can  make  a  neutral  or  infinite  wave  appear  as 
though  it  came  from  a  point  within  infinity,  or  vice  versa;  and 
by  placing  prisms  before  the  eye  we  can  thus  induce  an  exer- 
cise of  positive  or  negative  convergence,  for  the  purpose  of 
maintaining  single  binocular  vision,  according  to  the  position 
and  strength  of  the  i)rism.  If  the  [)rism  be  placed  before  tlu' 
eye  with  its  base  toward  the  tem[)oral  side,  i.  o.,  "base  out." 
the  rays  of  light  will  be  made  more  di\ergent,  and  the  eyes 
will  be  obliged  to  turn  inward  to  maintain  single  vision.  If  it 
be  placc-d  "base-  in"  the  rays  will  be  iii.uK'  inoir  conxcTL^cnt. 
and  the  eyes  will  ha\e  to  turn  out.  I'>y  means  of  numbered 
prisms,  therefore,  the  inti-nial  an«l  c.Nti'rnal  rectus  muscles  can 
be  forred  into  action  to  the  limit  i>t  tlieii  physiological  capacity, 
and  that  capacity  expressed  in  pri.sni  (lio]itres.     With  the  base 


PRISM  363 

up  or  down,  the  same  thing  can  be  done  in  respect  of  the  su- 
perior and  inferior  recti. 

Again,  in  virtue  of  its  deviating"  power,  a  prism  may  be 
made  to  diverge  or  converge  the  rays  of  light  entering  a  pair 
of  eyes  whose  visual  axes  are  in  a  permanent  state  of  obliquity, 
as  they  are  in  strabismus  or  manifest  heterophoria,  so  as  to 
equalize  and  compensate  for  their  obliquity,  i.  e.,  so  as  to  co- 
incide with  their  visual  axes ;  and  the  numbered  strength  of 
the  prism  which  accomplishes  this  will,  of  course,  express  in 
prism  dioptres  the  degree  of  imbalance  existing. 

Again,  the  same  prism  deviation  which  measures  ocular  im- 
balance by  equalizing  the  deviation  of  the  visual  axes  also 
remedies  the  condition  by  enabling  the  eyes  to  maintain  single 
binocular  vision  without  exercise  of  the  ocular  muscles,  and 
thus  serves  as  a  correction. 

The  above  represent  the  three  chief  uses  of  prisms  in  ophthal- 
mic practice.  There  are  other  auxiliary  uses.  Maddox 
enumerates  seven  applications  of  prisms  in  clinical  work,  as 
follows : 

(1)  To  measure  the  functional  minimum  of  convergence. 

(2)  To  measure  the  absolute  maximum  of  convergence. 

(3)  To  measure  the  relative  range  of  convergence. 

(4)  To   dissociate  convergence  and  accommodation. 

(5)  To  detect  vertical  deviations. 

(6)  To  measure  strabismus. 

(7)  To    determine    the    presence    or   absence   of   binocular 

vision. 

(8)  To  elicit  diplopia  in  suppression  of  the  false  image. 

(9)  To  relieve  excess  of  tonic  convergence. 

(10)  To  relieve  deficiency  of  tonic  convergence. 

(11)  To  relieve  excess  of  accommodative  convergence. 

(12)  To  relieve  deficient  accommodative  convergence. 

(13)  To  correct  diplopia  in  oculo-motor  paralysis. 

(14)  To  assist  in  curing  paralytic  diplopia. 

(15)  To  disguise  the  squint  in  an  amblyopic  eye. 

For  detailed  particulars  of  the  use  of  prisms  for  the  above 
purposes,  the  readier  is  referred  to  the  sections  on  Convergence, 
Heterophoria,  and  Strabismus. 


364  PRISM  DIOPTRE 

Prism  Dioptre,  ^llie  unit  of  prism  power  measurement.  One 
prism  dioptre  is  the  power  to  deviate  a  ray  of  light  1  in  100, 
i.  e.,  1  cm.  in  one  meter  distance.  Devised  by  Chas.  Prentice 
of  New  York. 

Prismoptometer.  An  instrument  for  testing  the  eye  muscles 
with  prisms. 

Prismosphere.     A  combined  spherical  lens  and  prism. 

Projection.  Geometrically,  this  term  implies  the  continuation 
of  a  line,  or  a  series  of  lines,  in  one  direction  or  the  other, 
under  precisely  the  same  spacial  conditions  that  they  already 
exhibit.  Thus,  if  two  lines  are  inclined  toward  each  other  at  a 
certain  angle  of  deviation,  we  can  project  them  in  the  direction 
of  their  inclination  to  a  meeting  point,  or  in  the  direction  of 
their  deviation  at  the  same  angular  deviation  that  they  already 
have. 

Physiological!},  the  word  signifies  the  tracing  of  a  sensa- 
tion by  the  brain  to  its  point  of  origin.  .Strictly  speaking,  as  a 
matter  of  pure  sense,  this  projection  cannot  go  beyond  the 
place  at  which  the  stimulus  entered  the  body.  All  so-called 
projection  beyond  that  point,  into  space,  is  not  genuine  projec- 
tion at  all,  but  cerebral  judgment,  arrived  at  by  means  of  other 
data  than  that  supplied  by  the  sense  itself. 

As  applied  to  light  and  optics,  this  implies  that  the  brain 
can  project  the  impression  made  by  light  falling  on  the  retina 
only  as  far  as  the  cornea.  Whatever  further  projection  into 
space  the  brain  performs,  or  thinks  it  performs,  is  a  matter  of 
mental  judgment,  based  upon  the  size  of  the  image,  the  use  of 
the  accommodation  and  convergence,  and  former  experiences. 
It  follows,  therefore,  that  beyond  the  C(jrnea  the  projection  of 
images  can  be  only  in  a  straight  line.  No  matter  where  the 
source  of  light,  i.  e.,  the  object,  may  really  he  in  space,  the 
origin  of  the  inii)ressi()n,  so  far  as  the  sense  of  \  ision  is  con 
cerned.  is  at  the  cornea,  where  the  li^ht  lirst  enters  the  eye. 
I'urtlu-r  projection  is  a  mere  continuation  of  the  liiu-.ir  projec- 
tion from  the  retina  to  the  cornea. 

When,  tlieref<ire,  light  wa\es  from  ;iii  oliject  are  made  to 
turn  corners  or  curves  before   the\    «iit»r   tlie   e\e,  as   in   the 


PRO-OPHTHALMOS  365 

case  of  prisms  and  mirrors,  the  sense  of  vision  projects  the 
image  back,  not  to  its  actual  place  in  space,  but  in  a  straight 
line  with  the  visual  axis.  The  angular  separation  of  the  actual 
object  and  the  projected  image  is  called  the  angle  of  false 
projection, 

Pro-Ophthalmos,     Bulging  of  the  eyeball  forward. 

Prothesis  Ocularis.     1  he  inserting  of  an  artificial  eye. 

Protractor.  A  de\'ice  for  centering  lenses  and  prisms.  There 
are  several  different  varieties  on  the  market.  Most  of  them, 
however,  consist  essentially  of  a  chart  with  either  radiating  or 
cross  lines,  which  are  \iewed  through  the  lens  or  prism;  the 
lens  or  the  chart  is  made  to  revolve  until  the  lines  are  seen 
unbroken,  in  the  case  of  cylindrical  lenses  and  prisms;  in  cen- 
tering spherical  lenses  the  movement  is  lateral,  until  the  same 
thing  is  attained. 

Pseudoblepsia.     Perverted  \ision. 

Pseudo-Glioma.  A  false  appearance  of  glioma,  due  to  a  pus  sac 
in  the  vitreous. 

Psorophthalmia.     Itchy  ulceration  of  the  eyes. 

Pterygium.  An  overgrowth  of  conjunctival  tissue  which  starts 
from  the  inner  canthus  and  creeps  over  the  conjunctiva  toward 
the  cornea.  (Jccasionally  it  encroaches  upon  the  cornea,  in 
which  case  it  interferes  with  vision  and  calls  for  surgical  re- 
moval. 

Ptilosis.     Falling  out  of  the  eyelashes. 

Ptosis.  Drooping  of  the  upper  eyelid,  usually  due  to  paralysis 
of  the  third  nerve. 

Puncta  Lacrimalia.  The  two  eminences  in  the  inner  part  of  the 
eyelids  where  the  tear  ducts  open  into  the  conjunctiva. 

Punktal  Lens.  A  development  of  the  deep  curve  lenses  (Toric 
and  Meniscus),  the  surfaces  of  which  are  curved  and  are  more 
nearly  perpendicular  to  the  line  of  sight  passing  through  the 
margin — separate    computation    for    the    power    of    each    lens 


366  PUNCTUM 

and  its  correction  must  be  followed.  Each  lens  is  ground  and 
polished  according  to  a  formula  specially  computed  for  the  par- 
ticular power  of  the  kind  of  lens. 

Punctum.     A  point. 

Punctum  Proximum.    The  near  point. 
Punctum  Remolum.     The  far  point. 

Pupil.  This  name  is  given  to  the  circular  aperture  in  the  dia- 
l)hragm  of  the  iris.  It  varies  in  size  continually,  enlarging 
when  the  iris  is  drawn  back  toward  its  base  and  becoming 
smaller  when  the  iris  is  drawn  centripetally  in  the  direction 
of  its  free  margin.  Both  dilation  and  contraction  of  the  pupil 
are  positive  muscular  phenomena,  due,  respectively,  to  the  in- 
nervation and  contraction  of  the  radiating  and  circular  muscles 
of  the  iris.  (See  Iris.)  The  former  is  innervated  through  the 
third  cranial  nerve  (motor  oculi),  by  way  of  the  ciliary 
ganglion,  and  the  latter  through  the  cervical  sympathetics, 
having  their  origin  in  the  cervical  ganglia  anterior  to  the 
spinal  cord,  which  tap  the  ascending  sensory  paths  of  the  cord. 

Because  of  this  double  nerve  connection  of  the  iris,  the  size, 
shape  and  behavior  of  the  pupil  furnish  extremely  valuable 
data,  in  health  and  disease,  as  to  the  state  of  the  body,  mani- 
fested through  the  sympathetic  and  the  cerebro-spinal  nervous 
systems,  of  which  only  a  brief  summary  can  be  given  here. 

In  health  the  shape  of  the  pupil  is  almost  perfectly  circular. 
Distortions  are  practically  always  due  to  local,  mechanical 
causes — the  dragging  of  the  iris  in  one  or  more  directions, 
either  by  reason  oi  its  being  stuck  down  to  the  iris  or  the 
cornea  by  exudates,  or  being  caught  in  a  wound  of  the  cornea 
or  limbus.  The  former  accident  occurs  in  iritis  and  other  in- 
llammatory  diseases  of  the  eye;  the  latter  as  a  complication 
of  operations. 

'JMie  size  of  the  pupil  in  health  is  a  difficult  thing  to  specify, 
because,  as  stated,  it  is  continually  varying  under  physiidogical 
stimuli.  What  may  he  calkd  its  al)sohite  or  constant  is  per- 
sonal idiosyncrasy.  Since  light,  which  is  the  principal  agt-nt 
of  stimulus  for  contraction,  is  a  more  or  less  constant  factor, 
i.  e.,  the  same  for  all  iii(li\  iduals,  it  is  to  the  action  of  the 
sympathetics,  or  dilators,  that  we  must  look  for  an  explanation 


PUPIL  367 

of  individual  variations  in  the  size  of  the  pupils.  In  general 
it  may  be  said  that  sensitive,  neurotic  persons,  whose  sym- 
pathetics  are  easily  excited,  and  react  readily,  have  larger 
pupils  than  more  phlegmatic  individuals. 

Much  more  important  are  the  variations  in  the  size  of  the 
pupils  due  to  abnormal  or  pathological  causes,  concerning 
which  the  following  general  rules  may  be  laid  down : 

Mydriasis  (dilation  of  the  pupil)  is  caused  by  any  condition 
which  (a)  paralyzes  either  central  or  peripheral  ending  of  the 
third  nerve,  or  the  short  ciliaries,  (b)  inhibits  the  functionation 
of  the  third  nerve  through  the  brain,  or  (c)  stimulates  the  sym- 
pathetics. 

Tumors  of  the  brain,  or  along  the  third  nerve,  toxines  of 
acute  infectious  diseases  (diphtheria,  scarlatina,  etc.)  and  the 
action  of  certain  drugs,  notably  atropine,  are  examples  of  the 
first  class.  Fevers,  comatose  conditions,  and  excited  states  of 
the  brain  are  instances  of  the  second  class.  Pain,  sensory  irri- 
tation and  abdominal  diseases  furnish  illustrations  of  the  third 
variety. 

Myosis  (contraction  of  the  pupil),  on  the  other  hand,  results 
from  influences  which  (a)  irritate  the  third  nerve,  or  (b)  de- 
press the  sympathetics. 

Photophobia,  from  whatever  cause,  meningitis,  and  certain 
drugs,  such  as  eserine  and  pilocarpine,  are  instances  of  the 
former;  spinal  diseases,  aneurisms  (by  pressure  on  the  nerves), 
and  opium  of  the  latter. 

Normally,  the  two  pupils  are  equal  in  size.  Inequality  is 
practically  always  due  to  interruption  of  the  motor-oculi  nerve 
path,  either  central  or  peripheral.  Central  causes  are  brain 
tumors,  syphilis,  tubercle  of  the  brain,  and  the  like.  Peripheral 
causes  are  injuries  to  the  eye  and  toxines  from  acute  infectious 
diseases.  Occasionally  one  pupil  is  dilated  because  of  pressure 
on  the  cervical  sympathetics  by  an  aneurism.  Diseases  which 
interfere  with  light  stimulus  of  the  retina  (cataract,  optic 
atrophy,  etc.)  will  also  produce  unilateral  dilatation. 

In  health  the  pupil  reacts  with  great  quickness  and  sensi- 
tiveness to  stimulus  on  both  sides  of  the  ner\'ous  circuit,  i.  e. 
to  light  and  to  sensory  stimulus.  A  sluggish  or  immobile  pupil 
may  be  due  to  adhesion  of  the  iris  to  the  lens  or  cornea,  or  to 


368  PUPILLOMETER 

paral3'sis  of  the  third  nerve,  central  or  peripheral.  Most  of  the 
latter  class  of  cases  result  from  syphilis. 

The  .\rgyll  Robertson  jnipil,  in  which  the  pupil  fails  to  con- 
tract in  response  to  light  but  contracts  upon  accommodation, 
occurs  in  locomotor  ataxia,  supposed  to  be  due  to  involvement 
of  Meinert's  fibres. 

Wernicke  pupillary  reaction  is  a  test  for  determining  the 
location  of  a  lesion  in  the  optic  tract,  employed  in  cases  of 
hemianopsia.  If  the  light  reflex  is  present  in  the  blind  half  of 
the  e\e  then  the  lesion  is  back  of  the  optic  thalamus;  if  not.  it 
is  in  front  of  this  ganglion;  because  at  that  point  the  reflex 
and  optic  paths  part  company. 

Hippus,  a  periodic  contraction  ami  dilatation  of  the  pupil.  i> 
seen  in  psychic  diseases,  mania,  hysteria,  etc. 

In  all  tests  designed  to  ascertain  the  reaction  of  the  pupil 
careful  attention  must  be  paid  to  the  presence  or  absence  of 
light,  according  as  the  examiner  wishes  light  to  lie  a  factor  in 
the  test  or  not. 

Pupillometer.  An  instrument  for  measuring  the  diameter  of  the 
l)upil.  There  are  many  such  devices.  Edgar  Browne's  con- 
sists of  a  series  of  graduated  circular  holes  in  an  oblong  i)iece 
of  wood  or  ivory,  each  one  notated  with  its  diametrical  meas- 
urement ;  the  hole  coinciding  with  the  pupillary  aperture  is 
easily  found.  Lawrence's  instrument  is  a  screw-sliding  bar, 
much  after  the  fashion  of  a  universal  monkey-wrench,  which  is 
opened  by  a  screw-movement  to  the  width  of  the  pupil.  Priest- 
ley .Smith's  de\ice  is  an  o])a(|ue  disc,  with  a  graduated  slit  in  it. 

Pupilloscope.  Pupilloscopy.  An  instrument  and  method  for 
niea>urinL;  the  puiMJlo-niotor  sensibility  of  the  retina  to  light. 

Pupillostatometer.  .\u  instrument  tor  measuring  the  di>tance 
betw  een  the  i)ui)ils. 

Purple,  Visual.  I  he  reddish  photo-eliemieal  substance  in  the 
rods  of  the  retina,  the  disintegration  of  which  constitutes  the 
])hysical  stimulus  of  vision. 

Pyophthalmia.     I'u^  in  tlu-  eye. 

Quadrant.      The  section  of  a  circle  containing  one  iij.;ht  angle. 


QUADRIGEMINAL  BODIES  369 

Quadrigeminal  Bodies.  A  group  of  four  nerve  ganglia  at  the 
mid-brain,  composing  a  portion  of  the  sub-station  generally 
known  as  the  optic  thalamus,  where  the  neurons  from  the 
optic  tract  are  relayed.  See  Optic  Tract  and  Physiology  of 
Vision. 

Radial.     Diverging,  as  rays,  from  a  centre. 

Radian.  An  arc  of  a  circle  equal  to  the  radius.  A  hundredth 
part  of  the  radian  is  a  centrad,  one  of  the  units  of  measure- 
ment of  the  deviating  power  of  a  prism.     See  Prism. 

Radiant  Energy.  A  form  of  energy  which  is  propagated  through 
the  medium  of  the  luminiferous  ether,  of  which  light  is  a  con- 
spicuous example. 

Radiation.     Divergence,  in  the  form  of  rays,  from  a  centre. 

Range  of  Accommodation.  The  distance  between  the  far  point 
and  near  point  of  accommodation.    See  Accommodation. 

Range  of  Convergence.  The  distance  between  the  far  point  and 
the  near  point  of  convergence.     See  Convergence. 

Ray.  The  geometrical  equivalent  of  a  wave  of  light,  showing 
its  linear  propagation.  When  we  wish  to  work  out  linear  cal- 
culations, we  use  the  ray.  When  Ave  are  working  with  curva- 
tures, we  use  the  wave.     Both,  of  course,  are  mere  symbols. 

Rectangle.  A  four-sided  geometrical  figure,  having  all  its  angles 
right  angles,  and  therefore  its  opposite  sides  equal  and  parallel. 
A  rectangle  is  said  to  be  contained  by  any  two  of  its  sides 
containing  one  of  its  right  angles. 

Rectus.  Literally,  straight.  Applied  to  all  those  muscles  of  the 
e\  e  which  move  the  eyeball  in  a  direction  straight  with  the 
Acrtical  or  the  horizontal  meridian.     See  Muscles. 

Red-Blindness,  Inability  to  distinguish  red  color.  It  is  the 
commonest  form  of  color-blindness. 

Reduced  Eye.  The  refractive  system  of  the  eye  based  upon  an 
imaginary  surface  having  the  same  net  dioptrism  and  the  same 


370  REFLECTION 

posterior  principal  focus  as  the  compound  refracting  system 
of  the  eye.     (See  Eye). 

Reflection.  The  turning  back  of  a  wa\  e  of  light  by  a  more  or 
less  polished  surface.  Reflection  occurs  in  accordance  with 
two  laws,  namely, 

1.  The  angles  which  the  incident  and  reflected  rays  make, 
respectively,  with  the  perpendicular  of  the  reflecting  surface, 
are  equal. 

2.  The  incident  and  reflected  rays  are  in  the  same  plane. 
For  practical  aspects  of  reflection  see  Mirror. 

Reflector.  A  device  for  reflecting  light,  usually  consisting  of  a 
polished  curved  surface. 

Reflex.  In  physiology  a  reflex  is  an  automatic,  or  unconscious, 
response  of  a  muscle  to  stimulus,  mediated  through  the  ner- 
vous system.  The  necessary  factors  to  a  reflex  are  (1)  a  stim- 
ulus, (2)  a  sensory  nerve  path  to  the  cord  or  brain,  (3)  an  arc, 
where  the  nerve  impulse  is  transferred  from  the  sensory  to  a 
motor  nerve  path,  (4)  a  motor  nerve  path,  to  carry  the  re- 
sponding impulse  to  a  muscle,  and  (5)  a  muscle,  to  contract 
in  response  to  the  stimulus. 

In  the  case  of  the  eyes  there  are  two  important  reflexes, 
which  ought  to  be  thoroughly  understood  by  those  who  deal 
with  them : 

(1)  The  light  reflex.  This  is  the  response  of  the  concentric 
muscles  of  the  iris  to  the  stimulus  of  light,  causing  the  pupil 
to  contract  when  light  is  thrown  upon  the  retina.  In  this  in- 
stance, the  stimulus  is  the  light,  the  sensory  nerve  path  is  the 
optic  tract  (q.  v.)  as  far  back  as  the  quadrigeniinal  bodies,  the 
arc  is  the  optic  thalamus,  the  motor  path  is  the  third  nerve,  and 
the  muscle  the  concentric  or  sphinctre  muscle  of  the  iris.  .Any 
interruption  to  this  round-trip  nerve  course  will  interfere  with 
the  integrity  of  the  light  reflex. 

(2)  The  accommodation  reflex.  This  is  the  response  of  the 
ciliary  and  sphinctre  iridis  muscles  to  the  call  for  a  clear  image. 
Mere  the  stimulus  is  an  indistinct  image,  i.  e.  a  confusion  circle 
of  imfocusscd  waves,  falling  upon  the  retina,  the  sensory  p.ith 
is  thi-  entire  optic  tract  (q.  v.)  clear  to  the  left  frontal  convolu- 
tion of  the   br.iiii,   the  ;irc  is  in   the   froiit.il   cittivolution,   the 


REFRACTING  MEDIA  371 

motor  path  is  from  the  frontal  lobe  to  the  fourth  ventricle  and 
thence  along  the  third  nerve  to  the  ciliary  ganglion,  thence  by 
sympathetics  to  the  ciliary  body,  and  the  muscles  are  the 
ciliary  and  the  sphinctre  iridis.  Any  interruption  of  this  course, 
at  any  point,  will  interfere  with  the  accommodation  reflex. 

The  light  reflex  is  impaired  or  destroyed  in  certain  well- 
known  diseases  of  the  nervous  system,  such  as  locomotor 
ataxia,  myelitis,  spinal  syphilis,  etc.  The  accommodation  is 
rarely  interrupted. 

When  we  put  atropine,  homatropine,  or  cocaine  in  the  eye, 
we  arbitrarily  interrupt  both  light  and  accommodation  reflexes, 
by  temporarily  paralyzing  the  endings  of  the  motor  nerve 
path  which  conduct  the  impulse  to  the  ciliary  and  iris  muscles. 

When  we  put  eserine  or  pilocarpine  in  the  eye,  we  artificially 
imitate  these  reflexes  by  keeping  up  an  irritation  of  the  endings 
of  the  motor  nerve  tracts. 

In  either  of  the  above  reflexes,  if  the  stimulus  be  applied  to 
one  retina  alone,  the  other  eye  will  respond.  This  is  known 
as  the  consensual  reflex.     (See  Physiology  of  Vision). 

Refracting  Media.  Any  bodies  or  substances  through  which 
light  passes  and  is  refracted  as  it  passes.  The  refracting  media 
of  the  eye  are  the  cornea,  the  aqueous  humor,  the  crystalline 
lens,  and  the  vitreous. 

Refraction.  When  a  wave  of  light,  moving  in  a  given  medium, 
falls  upon  the  surface  of  another  medium  of  different  density, 
if  the  normal  of  the  wave-curve  is  perpendicular  to  the  surface, 
it  passes  through  the  new  medium  unchanged;  but  if  the 
normal  of  the  wave-curve  be  other  than  perpendicular  to  the 
surface,  its  curvature  is  changed  as  it  enters  the  new  medium, 
and  the  direction  of  its  rays  altered.  If  the  waves  pass  from 
a  rarer  into  a  denser  medium,  the  rays  are  bent  toward  the 
perpendicular  of  the  surface ;  if  from  a  denser  into  a  rarer,  away 
from  the  perpendicular.  This  phenomenon  is  known  as  re- 
fraction. 

The  laws  governing  refraction  are  as  follows : 
(1)   Incident  and  refracted  rays  are  in  the  same  plane  with 
the  normal  at  the  point  of  incidence. 


372  REFRACTIONIST 

(2)  The  ratio  of  the  sines  of  tlie  angles  between  the  normal 
and  the  incident  and  refracted  rays,  respectively,  is  constant 
for  the  two  media,  varying  according  to  the  wave-length  of  the 
light.  I'his  constant  is  the  relative  index  of  refraction  of  the 
second  medium.  When  the  first  medium  is  a  vacuum,  the 
refractive  index  is  absolute.  The  refractive  index  of  air  is 
regarded  as  1. 

The  alteration  of  the  wave  depends  upon  the  ditterence  of 
velocity  with  which  it  is  propagated  through  the  new  medium. 
Light  waves  travel  in  air  at  the  rate  of  approximately  186,000 
miles  per  second;  in  crown  glass  their  \  elocity  is  reduced  to 
slightly  over  122,000  miles  per  second.  With  the  refractive 
index  of  air  expressed  as  1,  this  makes  the  refractive  index  of 
crown  glass  a  little  over  1.50,  and  this  ratio  is  geometrically 
expressed  by  the  ratio  of  the  sines  of  the  two  angles  as  stated 
above. 

The  change  is  due  to  a  turning  of  the  wave-front,  which,  in 
turn,  is  due  to  an  interruption  of  the  time  rhythm  in  the  pro- 
pagation of  the  lateral  elements  of  the  wave.  If  the  normal  of 
the  wave-curve  strikes  the  refracting  surface  perpendicularly, 
this  interruption  takes  place  symmetrically  on  both  sides  of 
the  normal,  so  that  no  turning  of  the  front  occurs;  but  if 
obliquely,  the  interruption  is  asymmetrical,  and  the  front  is 
turned,  much  as  the  front  of  a  company  of  marching  soldiers 
would  be  turned  by  crossing  a  river  oblicpiely. 

The  degree  of  refraction,  therefore,  depends  upon  the  degree 
of  difit'erence  in  the  densities  of  the  two  media,  and  the  oblicjuity 
with  which  the  normal  of  the  waves  strikes  the  surface. 

For  practical  considi  rations,  see  Lens.    Also  Light. 

Refractionist.  (Jne  who  is  skilled  in  nuasurin^  and  correcting 
encjrs  of  refraction  of  the  eye. 

Refractive,     rertaining  to  refraction. 

Refractometer.     An  instrununt  for  nieasuiing  refraction. 

Relative  Index  of  Refraction.  The  ratio  of  t)plical  density  borne 
by  one  substance  toward  another  substance  whose  density  is 
expressed  by  a  positi\e  number,  i.  e.  other  than  a  vacuum.  As 
com])ared  with  a  vruuuni,  the  index  is  said  to  be  absolute. 


REPOSITION  373 

Reposition.  Putting  back  into  the  normal  place — specifically 
applied  to  the  replacing  of  the  iris  in  cataract  operations. 

Retina.  The  so-called  third  tunic  of  the  eye  (although  histolog- 
ically it  is  the  first) — a  thin,  transparent  membrane,  which  is 
spread  out  over  the  inner  surface  of  the  chorioid  and  serves  as 
the  sensitive  film  for  the  camera  of  the  eye.  It  consists  in 
reality  of  a  flared  continuation  of  the  optic  nerve,  whose  end- 
ings are  distributed  over  the  retina  in  the  form  of  rods  and 
cones.  The  retina  is  well  supplied  with  blood  through 
branches  of  the  central  retinal  artery,  which  enters  with  the 
optic  nerve. 

A  little  to  the  temporal  side  of  the  optic  disc  is  a  small  cir- 
cular area,  devoid  of  vessels,  but  thickly  furnished  with  nerve 
endings,  which  is  the  most  sensitive  spot  in  the  retina.  Be- 
cause of  its  yellow  appearance  (due  to  the  choroid  showing 
through)  it  is  known  as  the  yellow  spot,  or  macula  lutea.  In 
the  centre  of  the  macu-la  is  a  still  more  sensitive  depression, 
called  the  fovea  centralis. 

The  retina  is  made  up  of  ten  layers,  from  within  outward  as 
follows : 

Internal  limiting  membrane 

Fibrous  layer  (nerve  fibres) 

Vesicular  layer  (nerve  cells) 

Inner  molecular  layer 

Inner  nuclear  layer 

Outer  molecular  layer 

Outer  nuclear  layer 

External  limiting  membrane 

Layer  of  rods  and  cones  (Jacobs'  membrane) 

Pigmentary  layer 
The  layer  of  rods  and  cones  is  the  important  layer  from  an 
optical  standpoint,  for  it  is  here  that  the  light  vibrations  are 
transformed  into  visual  impulses.  The  rods  contain  a  photo- 
chemical substance,  known  as  the  visual  purple,  which  gives 
to  the  retina  in  life  a  reddish  color.  A  few  of  the  cones  con- 
,  tain  visual  purple,  but  in  general  they  are  devoid  of  it.  This 
substance  is  decomposed  ("bleached")  by  light  waves,  each 
different  color  wave  having  a  different  bleaching  power,  set- 


374  RETINAL  REFLEX 

ting  up  a  visual  impulse  in  the  nerve  ending.  In  the  macula 
there  are  no  rods,  but  only  cones,  and  there  is  no  visual  purple. 
The  retina  is  attached  to  the  chorioid  only  at  the  anterior 
border  (ora  serrata)  and  at  the  optic  disc.  Occasionally  it 
becomes  partial!}^  detached. 

Retinal  Reflex.  The  glare  of  red  light  produced  by  the  light 
which  emerges  from  the  ])atient's  eye  in  retinoscopy.  .Mso 
known  as  the  pupillary  reHex. 

Retinitis.  Inflammation  of  the  retina,  characterized  by  excessive 
redness  and  increase  in  number  of  visible  vessels,  and  some- 
times tiny  hemorrhages.  There  is  usually  interference  with 
vision.  The  differentiation  between  the  various  types  of  retin- 
itis and  their  significance  Ijelong  to  medicine. 

Retinoscope.  An  instrument  constructed  on  the  same  principle 
as  the  ophthalmoscope  (See  Ophthalmoscope),  except  that  it 
usually  is  made  with  a  plane  retlecting  mirror,  or,  if  concave, 
not  so  concave  as  that  of  the  latter  instrument.  Its  object  is 
not  to  examine  the  details  of  the  fundus,  hut  to  obtain  a  fair 
general  ilhmiination  of  the  pupillary  area. 
h\»r  use  of  the  instrument  see  Retinoscopy. 


KirrixosciMTC. 


Retinoscopy.  An  (tl)j(iti\  e  inelhoil — in  fact,  the  onls  avail- 
able objective  nutlio(l  of  (ieterminiui;  the  refraction  tif  thr 
i-ye    by    certain    shadow    phenonuii,!    i)ro(hui'd,    obser\e<l    and 


RETINOSCOPY  375 

modified  by  means  of  an  instrument  called  the  retinoscope. 
The  procedure  is  also  known  as  skiascopy,  and  the  instrument 
as  a  skiascope. 

THE  RETINOSCOPE. 

The  retinoscope  itself  is  an  adaptation  of  the  principle  of 
the  ophthalmoscope  to  skiascopy.  The  ophthalmoscope  was 
the  original  instrument,  designed  by  Helmholz,  as  elsewhere 
related,  for  the  purpose  of  enabling  us  to  view  and  examine 
the  patient's  retina,  by  intercepting  the  light-waves  that 
emerge  from  the  patient's  pupillary  opening  and  causing  them 
to  enter  our  own  pupil.  It  was  not  until  several  years  later 
that  the  shadow  test  for  refraction  was  discovered,  and  the 
same  principle  applied,  in  a  modified  form,  to  this  procedure. 

The  retinoscope,  then,  consists  simply  of  a  small  circular 
mirror,  pierced  through  the  centre  with  a  sight-hole,  and 
mounted  on  a  stem  handle.  The  diameter  of  the  mirror  is 
variable,  according  to  the  taste  and  usage  of  the  operator, 
ranging  from  1.5  to  3  cm.;  the  diameter  of  the  sight-hole  also 
varies  from  1  to  2  mm.  The  mirror  may,  also,  be  either  plane 
or  concave ;  although  they  are  never  made  as  concave  as  an 
ophthalmoscope  mirror,  because  the  same  concentration  of 
light  is  not  desired.  The  plane  mirror  is  most  commonly 
employed,  and  will  be  assumed  in  the  following  description  of 
retinoscopy. 

By  means  of  this  circular  mirror,  light  is  thrown  by  reflec- 
tion from  some  source  of  illumination  into  the  patient's  pupil 
and  onto  his  retina.  The  source  of  light  may  be  any  form  of 
lamp — gas,  electric,  or  even  oil — but  should  be  so  arranged  as 
to  have  the  examining  room  in  as  nearly  total  darkness  as 
possible.  Directions  for  carrying  out  this  technique  will  be 
given  later  on.  Modern  retinoscopes  are  made  with  their  own 
electric  lamps  attached  to  the  handle-stem  close  to  the  mirror 
— self-illuminating  retinoscopes — and  these  are  much  the  best 
to  use. 

The  light  being  thus  thrown  by  reflection  onto  the  patient's 
retina,  emerges  again  from  the  patient's  eye  through  his 
pupillary  opening.  It  was  formerly  taught  that  the  light  was 
re-reflected  by  the  retina  ;  and  we  still  speak  of  the  appearance 


376  RETINOSCOPY 

of  the  emergent  light  as  the  "pupiUary  reflex."  It  is  now  pretty 
generally  considered,  however,  that  the  patient's  retina  itself 
becomes  a  source  of  illumination ;  and  the  emergent  light  is 
regarded  as  originating  in  the  patient's  retina. 

If  the  retinoscope  now  be  held  as  nearly  as  possible  in  a 
horizontal  plane  with  the  patient's  pupil,  and  not  too  far  away, 
and  the  operator's  eye  be  applied  to  the  sight-hole  at  the  back 
of  the  mirror,  (taking  care  to  keep  the  reflected  light  thrown 
into  the  patient's  pupil),  the  operator's  eye,  behind  the  i)eep- 
hole,  will  intercept  the  light  emerging  from  the  patient's  eye. 
and  obtain  a  view  of  his  retina.  It  will  not  be  a  detailed  view; 
the  concentration  of  light,  and  the  optical  relations  of  the 
instrument,  do  not  afYord  a  detailed  view  of  the  fundus,  nor  is 
such  a  view  desirable.  It  is  seen  merely  as  a  red  glare  ;  tech- 
nically known  as  the  red  reflex. 

Considerable  practice  is  needed  to  be  able  to  execute  the 
manoeuvers  necessary  to  obtaining  a  good  "reflex."  And  not 
until  this  has  become  easy  to  the  operator  can  he  devote  the 
requisite  attention  to  producing  and  interpreting  the  shadow 
phenomena  which  constitute  the  essence  of  retinoscopy.  At 
the  end  of  this  section  will  be  found  a  set  of  instructions  and 
suggestions  for  carrying  out  the  technique  of  the  test,  to 
Avhich  tlu-  reader  is  referred. 

THE  MECHANICS   OF  THE   RETINOSCOPE. 

Before  proceeding  to  discuss  the  i)rinciples  an<l  technique 
of  retinoscopy,  it  is  important  that  the  student  should  get  a 
clear  conception  of  the  working  mechanics  of  the  n-tinoscope. 
We  have  already  described  the  instrument  itself;  and  shown 
how  it  enables  us  to  view  the  light  which  comes  from  the 
patient's  retina.  It  now  behooves  us  to  ask  ourselves,  con- 
cerning the  use  of  the  instrument  in  retinoscopy,  what  it  is 
expected  to  do,  and  how  it  is  expected  to  do  it. 

Suppose  yourself  to  be  standing  in  the  middle  of  a  circular 
turret,  totally  dark  except  for  one  small  round  port-hole  cut  in 
the  wall.  'Jhis  port-hole  appears  as  a  small  round  disc  of 
light.  And  suppose  that,  wliili'  you  arc  ga/ing  at  the  port 
bole,  the  whub-  turret  starts  to  revoKc.  turning  \  ou  with  it. 
What  ii.ippc-ii^  to  the  disc  of  light? 


RETINOSCOPY  377 

As  the  turret  revolves,  a  crescent-shaped  shadow  seems  to 
move  across  the  disc  from  the  side,  and  in  the  direction  that 
the  turret  is  revolved.  The  shadow,  or  dark  area,  is  in  reahty 
the  round  edge  of  the  wall  around  the  port-hole  that  has  come 
between  your  eye  and  a  portion  of  the  light;  but  as  you  cannot 
distinguish  the  wall,  it  appears  only  as  a  shadow  moving 
across  the  disc.  Or  it  would  be  just  as  correct  to  say  that  the 
disc  of  light  itself  is  moving  in  the  same  direction  as  the 
re\'olution  of  the  turret ;  but  it  appears  to  remain  stationary 
while  the  dark  area  following  it  seems  to  move. 

Suppose,  now,  that  you  see  this  same  thing  happen,  but  that 
your  back  is  turned  to  the  port-hole,  and  you  witness  the 
affair  in  a  mirror.  Then  the  shadow  will  seem  to  move  across 
the  disc  in  the  opposite  direction.  Everything  is  really  hap- 
pening in  all  respects  as  before,  but  the  effect  of  the  reflection 
in  the  mirror  is  to  reverse,  laterally,  the  light  rays  that  reach 
you  from  the  disc,  and  make  it  appear  that  the  movements  are 
reversed. 

Suppose,  once  more,  that  instead  of  being  in  the  middle  of 
the  room,  watching  the  port-hole,  your  face  is  right  at  the 
external  opening  of  the  port-hole  itself.  Then,  no  matter  if 
the  turret  revolves  on  its  pivot,  in  either  direction,  your  view 
of  the  light  is  uninterrupted,  and  you  hardly  know  the  tower  is 
turning. 

This  illustrates,  in  a  rough  fashion,  something  of  the  mechan- 
ical state  of  affairs  when  the  pupillo-retinal  area  of  light  is 
viewed  through  the  retinoscope,  and  the  instrument  is  rotated 
for  skiascopic  purposes. 

If  the  pencil  of  light  rays  which  emerge  from  the  patient's 
eye  and  stream  through  the  sight-hole  of  the  retinoscope  are 
as  yet  unfocussed,  (either  divergent  or  parallel),  as  long  as 
the  instrument  is  held  still,,  in  a  right  plane  between  the 
operator's  eye  and  the  patient's,  the  pupil  will  be  seen  as  a 
circular  disc  of  red  light;  but  on  turning  the  retinoscope  and 
the  operator's  head  with  it,  swivel-fashion,  the  oblique  tilt  of 
the  mirror  outward  will  cut  off  some  of  the  rays  on  that  side 
from  entering  the  operator's  eye,  and  a  shadow  will  seem  to 
move  across  the  pupil  from  that  side. 


378  RETINOSCOPY 

If  the  pencil  of  emergent  rays  have  already  focussed  before 
they  reach  the  retinoscope,  then  the  rays  which  come  through 
the  sight-hole  will  be  reversed  not,  as  in  the  turret  illustra- 
tion, laterally  reversed  by  reflection,  but  totally  reversed  by  re- 
fraction. The  result  is  that,  although  precisely  the  same  thing 
happens  on  rotating  the  instrument  as  happened  in  the  pre- 
N'ious  instance,  the  shadow  will  appear  to  come  from  the 
opposite  side  of  the  ])U])il,  and  move  across  the  pupil  in  the 
opposite  direction. 

As  a  matter  of  fact,  there  is  no  shadow  in  the  eye,  or  even 
on  the  cornea,  in  either  case;  it  is  that  the  ol)lique  tilt  of  the 
mirror  toward  the  path  of  the  emergent  rays  prevents  some  of 
them  from  being  intercepted  by  the  operator's  eye.  The 
illumined  area  simply  goes  out  of  vision  on  that  side.  The  fact 
that  you  can  still  see  the  patient's  cornea  and  iris  in  the  area 
of  shadow  (just  as  you  can  see  the  rest  of  his  face)  is  merely 
due  to  the  general  illumination  in  the  room,  which  cannot  be 
quite  prevented.  You  cannot  see  the  retina  in  the  darkened 
area. 

Finally,  if  the  emergent  rays  passing  through  the  sight-hole 
are  exactly  focussed  at  the  sight-hole,  (or,  what  is  practically 
the  same  thing,  at  or  near  the  operator's  nodal  i)oint),  then 
they  are  a  group  of  radial  points,  and  a  slight  rotation  of  the 
mirror  will  not  cut  any  of  them  ofT  from  the  observer's  eye. 
We  are  then,  so  to  sj)eak,  in  the  i)osition  of  having  our  faces 
at  the  outer  opening  of  the  turret  port-hole.  Whichever  way 
we  rotate  the  mirror,  there  will  be  no  shadow  moxing  across 
the  jjupil. 

In  the  first  of  these  three  events  the  shadow  is  technically 
said  to  move  "with  the  mirror"  (i.  e.  with  the  retinoscope); 
and  in  the  second  case,  "ag.iinsl  ihe  niiircir."  In  the  last  in- 
stance, the  focal  jjoint  at  the  sight-hole  of  the  mirror  is  said 
to  be  the  "point  of  reversal,"  because  if  the  instrument  be  held 
a  little  within  this  point  (i.  e.  a  little  nearer  to  the  i>atient's 
eye),  there  will  be  a  shadow  moving  "with,"  and  if  it  be  held  a 
little  outside  this  i)oinl,  (i.  e.  a  little  further  from  the  patient's 
eye),  there  will  be  ;i  shadow  moxing  "against."  In  fact,  if 
llierc  be  any  doubt  in  the  o|ier;itor's  mind  ;is  to  whether  he  has 


RETINOSCOPY  379 

obtained  the  point  of  reversal  or  not,  it  is  a  good  idea  to  test 
it  in  this  way. 

CHARACTER  OF  THE  SHADOWS. 

If  we  consider  the  emergent  rays,  treated  with  the  objective 
lens,  as  a  cone  of  light,  whose  base  is  the  pupillary  area  and 
whose  apex  is  the  point  of  reversal,  then  the  nearer  we  get  to 
the  apex  of  the  cone  the  smaller  is  the  cross-section  area  into 
which  the  rays  are  packed,  and  the  more  of  these  rays  will  be 
intercepted  by  a  given  depth  of  cut-off. 

In  high  errors  of  refraction  the  point  of  reversal  is  relatively 
far  from  the  mirror  of  the  retinoscope;  in  low  errors  it  is  rela- 
tively close  to  it.  In  high  errors,  therefore,  the  cut-off  of  the 
edge  of  the  sight-hole  intercepts  a  relatively  small  number  of 
the  emergent  rays,  while  in  low  errors  the  same  cut-off  inter- 
cepts relatively  many  of  these  rays.  As  a  consequence,  in  high 
errors  the  pupillary  shadows  are  indistinct,  and  appear  to  move 
slowly;  in  low  errors  they  are  sharply  defined,  and  seem  to 
move  rapidly.  The  nearer  we  approach  correction,  the  sharper 
the  shadows  become  and  the  more  rapidly  they  seem  to  move. 

PRINCIPLES  AND  PRACTICE  OF  RETINOSCOPY. 

The  principle  of  retinoscopy  is  the  principle  of  conjugate 
foci.  In  any  lens  system  the  image-point  is  conjugate  with 
the  object-point,  and  the  two  points  are  relationally  inter- 
changeable. In  the  eye  at  rest,  the  retina  is  conjugate  with  its 
far  point;  and,  if  the  eye  be  emmetropic,  so  that  its  retina  lies 
in  the  plane  of  its  posterior  principal  focus,  it  is  conjugate  with 
infinity.  Per  contra,  if  the  retina  of  the  emmetropic  eye  be 
tlie  source  of  light,  the  emergent  waves  will  be  neutral  (infin- 
ite) waves. 

If  an  eye  be  hyperopic,  then  the  retina  of  that  eye  at  rest  is 
conjugate  with  a  point  beyond  infinity,  and  emergent  waves 
are  divergent  (positive)  waves  to  the  extent  that  the  eye  is 
hyperopic.  If,  on  the  other  hand,  the  eye  is  myopic,  its  retina 
is  conjugate  with  a  point  within  infinity,  and  the  emergent 
waves  are  convergent  (negative)  to  the  extent  of  the  myopia. 

If,  then,  some  means  can  be  applied  to  determine  whether 
the  emergent  waves  from  the  static  eye  are  neutral,  positive  or 
negative,  and  to  measure  the  degree  of  their  divergence  or 
convergence,  we  can  thereby  determine,  and  measure,  and  cor- 


380  RETINOSCOPY 

rect  the  refraction  of  the  eye.  Retinoscoj)y  furnishes  us  the 
means. 

We  place  before  the  patient's  eye  a  convex  lens  of  known 
focal  power,  say  a  2  D.,  directing  him  to  look  into  infinity. 
The  focal  length  of  this  lens  is  50  cm.,  hence  if  the  waves 
emerging  from  his  eye  are  neutral  they  will  be  brought  to  a 
focus  at  a  point  50  cm.  in  front  of  the  lens.  If,  then,  we  hold 
the  sight-hole  of  the  retinoscope  at  this  point  and  look  through 
it,  we  shall  see  a  focussed  image  of  the  retina,  and  whichever 
way  w-e  slightly  twirl  the  mirror  we  shall  continue  to  see  the 
circular  glare  of  the  pupil — there  will  be  no  moving  shadow. 

Assume,  now,  that  the  eye  is  hypcropic,  so  that  the  emergent 
waves,  instead  of  being  neutral,  are  divergent.  Then  the  2  D. 
convex  lens  will  not  be  sufificient  to  focus  these  waves  at  its 
focal  length  of  50  cm.,  and  if  we  hold  the  mirror  at  this  distance 
the  light  which  enters  the  sight-hole,  instead  of  being  a  group 
of  focal  points,  will  be  a  group  of  diffusion  circles  of  unfocussed 
waves.  As  long  as  we  look  straight  through  the  sight-hole  we 
shall  see  the  circular  image  of  the  retina — not  clear  and 
bright,  as  in  the  case  of  the  focussed  image,  but  blurred  to  a 
dull  red — but  the  slightest  rotation  of  the  mirror  in  either 
direction  will  produce  a  shadow  moving  "with  the  mirror." 

If  we  now  find  a  plus  lens  which,  added  to  the  2  D.  already 
before  the  eye,  will  serve  to  just  bring  the  emergent  waves  to 
a  focus  at  the  50  cm.  point,  the  shadow  will  no  longer  appear, 
for  we  shall  again  be  viewing  a  focussed  image  of  the  retina. 
(Incidentally,  the  appearance  of  the  image  will  also  become 
bright  yellow-red).  This  is  technically  called  "abolishing,  or 
neutralizing  the  shadow."  The  additional  i)lus  lens  by  which 
it  was  accomplished  manifestly  neutralized  the  ilivergcnce  of 
the  emergent  waves  and  rcnderid  them  neutral,  thus  enabling 
the  2  D.  objective  lens  to  focus  them  at,  its  posterior  principal 
focus.  This  additional  lens  (known  as  the  neutralizing  lens) 
is  therefore  the  measure  of  the  jiatient's  hyperopia,  and  its 
correction. 

Again,  assume  that  the  eye  is  myopic,  so  that  the  enuMgent 
waves,  instead  of  being  neutral,  are  convergent.  In  this  case 
the  2  D.  coinex  lens  will  focus  tlirse  waves  in  a  less  distance 
than  5(J  cm.,  so  tliat  if  we  bold  our  retiuo^cope  ;it  the  50  cm. 


RETINOSCOPY  381 

distance  the  light  that  enters  the  sight-hole  will  be  a  group  of 
diffusion  circles  of  reversed  waves.  Rotation  of  the  mirror  will 
in  this  case  produce  a  shadow  moving  "against  the  mirror." 

We  may  proceed  to  find  a  concave  or  minus  lens  which, 
when  added  to  the  objective  of  2  D.  already  before  the  eye, 
will  delay  the  focussing  of  the  emergent  waves  just  sufficiently 
to  make  them  focus  at  50  cm.  and  abolish  the  shadow.  It  is 
evident  that  this  minus  lens  exactly  neutralized  the  converg- 
ence of  the  emergent  waves  and  rendered  them  neutral,  so  that 
th  2  D.  objective  lens  was  able  to  focus  them  at  its  posterior 
principal  focus.  This  minus  lens,  therefore,  is  the  measure  of 
the  patient's  myopia,  and  its  correction. 

Summarizing,  then,  the  general  rationale  of  retinoscopy  may 
be  briefly  expressed  as  follows : 

1.  We  seek  to  determine  whether  the  emergent  waves  from 
the  static  eye  are  neutral,  positive  or  negative. 

2.  For  this  purpose  we  proceed  to  force  these  waves,  by 
means  of  lens  power,  to  focus  at  a  given  distance  from  the 
eye, — in  other  words,  to  establish  an  arbitrary  conjugate  focus 
with  the  patient's  retina, — and  from  the  amount  of  lens  power 
needed  to  do  this  we  calculate  the  curvature  of  the  emergent 
waves. 

3.  The  amount  of  positive  or  negative  curvature  thus  dem- 
onstrated in  the  emergent  waves  represents,  respectively, 
hyperopia  or  myopia. 

The  method  for  carrying  out  the  procedure  may  be  sum- 
marized as  follows : 

1.  We  place  before  the  patient's  static  eye  a  convex  lens  of 
known  dioptrism,  and  work  with  our  retinoscope  at  the  prin- 
cipal focal  point  of  this  lens. 

2.  If  on  rotating  the  mirror  no  shadow  is  seen  moving  across 
the  patient's  pupil,  his  retina  is  conjugate  with  the  retinoscope; 
the  emergent  waves  are  neutral ;  the  eye  is  emmetropic. 

3.  If  there  appears  a  shadow  moving  "with"  the  mirror,  the 
patient's  retina  is  conjugate  with  a  point  beyond  the  retino- 
scope; the  emergent  waves  are  divergent;  the  eye  is  hyperopic. 
The  additional  convex  lens  which  makes  the  retina  conjugate 
with  the  mirror,  and  abolishes  the  shadow,  is  the  measure  of 
the  hyperopia. 


382  RETINOSCOPY 

4.  If  a  shadow  is  seen  moving  against  the  mirror,  the  conju- 
gate focus  is  between  the  retinoscope  and  the  patient's  eye;  the 
emergent  waves  are  convergent ;  the  eye  is  myopic.  The  addi- 
tional concave  lens  which  makes  the  retina  conjugate  with  the 
mirror,  and  abolishes  the  shadow,  is  the  measure  of  the  myo- 
pia. 

The  above-described  technique  is  probably  the  simplest,  as 
it  is  the  most  commonly  used.  It  has  two  practical  advan- 
tages: (1)  the  least  degree  of  hyperopia  or  myopia  is  at  once 
apparent  in  the  existence  and  movement  of  a  shadow,  (2) 
when  the  point  of  reversal  is  obtained,  no  calculations  have  to 
be  made,  the  neutralizing  lens  representing  the  error. 

There  are,  however,  other  methods  of  procedure  for  those 
who  prefer  them,  both  with  and  without  an  objective  lens. 
Some  operators  prefer  to  employ  an  objective  lens  only.  By 
this  method,  instead  of  forcing  the  point  of  reversal  on  to  the 
mirror,  the  operator  takes  his  mirror  in  search  of  the  point  of 
reversal,  and  having  found  it,  calculates  tiie  refractive  error 
by  the  difference  between  where  the  point  of  reversal  ought  to 
be  and  where  it  is.  If  p  represents  the  jiroper  point  of  re- 
versal, and  p^  the  point  of  reversal  as  found,  then, 
1  1 
==  error 

P        P' 
Thus,  if  with  a  plus  2  D.  lens  before  the  eye  the  point  of 

reversal  is  found  at  50  cm.  the  eye  is  emmetropic,  because  the 

point  ought  to  be  at  50  cm.,  so  that  we  have 

1         1 

..=^0      .50 
But  it,  with  lliis  ol)jcctive,  the  point  ol  rc\crsal  is  found  at  1 
meter,  then, 

1         1 
=.    1  I). 

..SO        1 

The  eye   is   1    I),  hypcropic.     ( >r  il,   with   the   same   lens,   the 

point  of  reversal  is  found  at  25  cm.,  tlu-ii. 

1         1 

^^—2  1 ). 

.50      .25 
The  eye  is  myopic  2  1"). 


RETINOSCOPY  383 

This  method  also  has  its  elements  of  technical  simplicity, 
chief  of  which  is  that  the  operator  does  not  have  to  keep 
changing  lenses.  Having  placed  the  objective  lens,  he  begins 
at  some  convenient  distance — say  a  meter,  to  shadow  the  eye. 
If  he  happens  immediately  to  find  the  point  of  reversal  (no 
shadow),  all  that  remains  is  to  make  the  calculation.  If  he 
finds  a  shadow  moving  with  the  mirror,  he  changes  the  objec- 
tive lens  for  a  stronger  one,  until  the  shadow  moves  against 
the  mirror.  Then  he  moves  his  mirror  gradually  in  toward  the 
patient's  eye  until  he  finds  the  point  of  reversal,  measures  the 
distance  from  the  patient's  eye,  and  makes  his  calculation 
against  the  focal  length  of  the  objective  lens,  as  shown  above. 

Still  another  method  is  to  work  at  a  fixed  distance,  without 
an  objective  lens  at  all,  but  only  a  neutralizing  lens.  In  this 
case,  the  operator  simply  finds  the  lens  which  will  abolish  the 
shadow  by  bringing  the  point  of  reversal  on  to  his  mirror.  If, 
when  he  starts  to  shadow,  the  shadow  is  moving  with  the 
mirror,  it  will  require  a  convex  lens  to  neutralize ;  if  it  is  mov- 
ing against  it  will  require  a  concave  lens.  Having  neutralized 
the  shadow,  he  then  subtracts  from  the  neutralizing  lens  the 
reciprocal  of  the  distance  at  which  he  worked — called  the 
working  distance  equivalent — and  the  remainder  represents 
the  refractive  error. 

Thus,  if  the  operator  works  at  50  cm.,  and  finds  the  shadow 
moving  with  the  mirror,  and  it  requires  a  plus  3  D.  to  neutral- 
ize the  shadow,  then, 

1 

3 =1  D. 

.50 
The  eye  is  1  D.  hyperopic.    Or  if,  working  at  the  same  distance, 
the  shadow  is  seen  to  move  against  the  mirror,  and  it  takes  a 
minus  2  D.  to  abolish  the  shadow,  then, 

1 

—3 =  — 5D. 

.50 
The  eye  is  5  D.  myopic.     Or,  again,  if,  at  tTie  same  distance, 
the  shadow  is  seen  to  move  with,  and  it  takes  a  plus  1  D.  to 
abolish  the  shadow,  then, 


384  RETINOSCOPY 

1 

1 ==— 1  D. 

.30 
The  eye  is  1  D.  myopic. 

As  will  1)6  seen,  this  method  really  amounts,  in  the  last 
analysis,  to  the  same  thing  as  the  method  first  described,  the 
objective  power  l)eing  figured  in  with  the  neutralizing  power 
and  then  figured  out  again,  instead  of  being  placed  before  the 
eye  in  the  form  of  a  lens  at  the  outset,  and  never  figured  into 
the  neutralizing  powder. 

RETINOSCOPY  IN  ASTIGMATISM. 

In  applying  retinoscopy  to  the  detection  and  estimation  of 
astigmatism,  we  have  only  to  bear  in  mind  that  it  is  still  a 
question  of  conjugate  foci,  and  that  we  are  dealing  with  the 
same  focal  conditions  on  the  anterior  side  of  the  ocular  lens 
system  that  exist  on  the  posterior  side.  In  the  definition  and 
description  of  astigmatism,  it  is  shown  that  the  two  chief  me- 
ridians of  the  eye  ha\e  dift'erent  refracting  power,  and  there- 
fore two  posterior  principal  foci,  both  situated  on  the  princi- 
pal axis,  one  more  posterior  than  the  other — or  one  more  an- 
terior than  the  other,  it  makes  no  difi^erence  how  we  state  it. 
Precisely  the  same  thing  is  true  of  the  anterior  focal  relations 
of  an  astigmatic  eye — its  two  chief  meridians  have  two  dif- 
ferent anterior  principal  foci,  two  diflferent  sets  of  conjugate 
foci,  and  therefore  two  jxiints  of  re\ersal.  one  more  anterior 
than  the  other. 

In  shadowing  an  astigmatic  eye.  then,  we  ha\  e  to  shadow 
two  difTerent  ocular  lens  systems,  corresponding  to  the  two 
chief  meridians,  in\-olving  two  com])lete  operations  by  any 
one  of  the  methods  already  described  ;  determine  the  refraction 
of  each  meridian  scparatel\'  ;  an<l  i  alcuiate  llu-  astigmatism  li\ 
the  difference  between  the  two  refractions. 

The  general  rule  of  procedure  is  very  simple.  In  the  casi- 
of  a  non-astigmatic,  spherical  eye  it  is  immaterial,  of  course, 
in  which  meridianal  direction  we  rotate  the  mirror,  since  the 
curvature  of  the  eye  is  the  same  in  all  of  its  meridians,  and 
the  shadow,  if  there  be  one,  will  behasi'  the  same  in  e\er\ 
meridian,  in  the  case  of  an  astigmatic  eye,  we  have  to  ascer- 
tain tiie  axes  of  the  chief  meridians,   and   do  our  shadowing. 


RETINOSCOPY  385 

i.  e.,  rotate  our  mirror,  at  right  angles  to  these  two  meridians, 
where  the  directions  of  greatest  and  least  refraction  lie. 

DETERMINING  THE  CHIEF  MERIDIANS. 

The  crux  of  the  problem  is  the  determination  of  the  axes  of 
the  two  chief  meridians.  Fortunately,  this  is  not,  as  a  rule, 
very  difficult ;  indeed,  in  most  cases  it  determines  itself.  We 
start,  in  every  case,  to  shadow  the  horizontal  meridian ;  i.  e., 
we  rotate  our  mirror  horizontally,  from  side  to  side.  If  there 
is  no  astigmatism,  and  the  eye  is  spherical,  the  shadow  (sup- 
posing there  to  be  a  shadow)  has  a  crescentic  edge  which  lies 
vertically  on  the  pupil.  If  the  eye  is  astigmatic,  the  edge  of 
the  shadow  will  usually  (not  invariably)  be  straight,  and  if 
the  astigmatism  is  an  oblique  one,  it  will  he  tipped  in  the 
direction  of  one  of  the  chief  meridians. 

Thus  we  are  advised  at  the  very  outset  of  the  existence  of  an 
oblique  astigmatism,  and  of  the  axis  of  one  of  the  two  chief 
meridians.  We  immediately  proceed  to  do  our  shadowing  at 
right  angles  to  this  axis  (as  shown  by  the  edge  of  the  shadow) 
until  we  abolish  the  shadow  in  that  meridian.  The  axis  of  the 
other  chief  meridian  is,  of  course,  at  right  angles  to  the  first, 
and  we  therefore  next  proceed  to  do  our  shadowing  across 
this  opposite  meridian  until  we  abolish  the  shadow  there  also. 

If,  however,  the  eye  has  a  right  astigmatism,  in  which  the 
two  chief  meridians  lie  vertically  and  horizontally  respectively, 
as  we  start  to  shadow  horizontally  there  will  be  nothing  to  tell 
us  of  the  existence  of  astigmatism  except  perhaps  the  straight 
edge  of  the  shadow.  In  this  case  we  must  simply  proceed  to 
abolish  the  shadow  in  this  horizontal  meridian.  When  we 
get  this  meridian  almost  neutralized,  so  that  the  light  area 
begins  to  change  to  a  yellow  color,  a  series  of  lines  of  reddish- 
brown  light  will  appear,  lying  horizontally  across  the  pui)il, 
seeming  to  shine  through  from  behind.  This  appearance, 
known  as  the  "astigmatic  band,"  indicates  astigmatism,  and 
is  due  to  the  fact  that  while  one  meridian  is  now  almost  fo- 
cussed,  the  other  is  yet  unfocussed. 

Whether  we  see  any  or  all  of  these  signs,  or  none  of  them, 
the  presence  of  a  right  astigmatism  can  very  easily  be  ascer- 
tained  if,  when   we  get  the   horizontal  meridian   neutralized. 


386  RETINOSCOPY 

we  shadow  the  vertical  meridian  with  the  same  neutralizing 
correction.  If  that  also  is  neutralized,  then  there  is  no  astig- 
matism :  but  it  there  is  still  a  shadow  moving  in  that  meri- 
dian, there  is  an  astigmatism,  and  we  must  proceed  accord- 
ingly. This  holds  good.  also,  with  regard  to  oblique  astig- 
matism ;  and  it  is  an  excellent  rule  never  to  regard  our  skia- 
scopic  job  as  finished  until  we  have  shadowed  the  eye  this  way 
and  that,  to  see  if  any  meridian  still  remains  uncorrected. 

THE  ASTIGMATIC   BAND. 

Many  authors  and  practitioners  greatly  emphasize  the  im- 
portance of  the  astigmatic  band,  and  rely  upon  it  for  deter- 
mining the  axis  of  one  of  the  principal  meridians.  It  has  al- 
ready been  stated  that  its  appearance  is  due  to  the  fact  that 
one  meridian  is  corrected  and  the  other  is  not.  Jackson  ex- 
plains the  phenomenon  on  the  ground  that  "'the  retinal  image 
will  be  more  magnified  in  the  direction  of  the  principal  meri- 
dian to  which  the  nearer  point  of  re\ersal  belongs."  Every 
point  of  light  in  that  meridian  then  ajjpears  as  a  line  of  light 
running  in  the  direction  of  the  jjrincipal  meridian  whose  point 
of  reversal  is  at  the  oj)erator's  eye. 

If  this  astigmatic  band  is  to  be  made  the  chief  factor  in 
dift'erentiating  the  axes  of  astigmatism,  special  care  must  be 
exercised  in  technique  to  bring  it  out  as  clearly  as  possible. 
Jackson  has  laid  down  the  rule  that  "the  band-like  appearance 
is  most  perfectly  de^■eloped  when  the  observer's  eye  is  at  the 
point  of  reversal  of  one  i)rincipal  meridian  and  the  imme- 
diate source  of  light  is  at  the  point  of  reversal  of  the  other 
meridian." 

Manifestly,  this  dual  condition  cannot  be  attaineil  with  the 
use  of  the  self-illuminating  retinoscope,  since  the  source  of 
light  in  that  instrument  is  practically  fixed  at  a  point  very 
near  to  the  mirror,  and  even  the  slight  ui)-and-down  play 
that  is  .'illowetl  tlie  l.inip  in  somi-  instrunii'uts  is  not  sufficient 
to  afford  any  api)reciable  ad.iptation  tt)  Jackson's  rule.  In 
order  to  make  the  most  of  the  .istigm.-itic  band,  therefore,  the 
operator  must  ni.il^c  use  of  an  indi'pendent  soiu"ci'  of  lii;ht. 
which  must  be  so  arranged  tl):it  he  c;in  nio\  e  it  tow.ird  and 
.'iw.'iv  from  his  mirror  at  will. 


RETINOSCOPY  387 

W  ith  this  arrangement  at  hand,  as  soon  as  the  principal 
meridian  that  he  is  shadowing  becomes  neutraHzed,  he  should 
push  the  lamp  away  from  his  mirror,  or  bring  it  nearer,  as 
necessary,  until  it  approximates  the  conjugate  focus  of  the  un- 
corrected meridian,  when  the  astigmatic  band  will  stand  out 
clear  and  distinct,  and  the  true  axis  of  the  astigmatism  be 
re^•ealed. 

CALCULATING  THE  ERROR. 

Having  found,  by  skiascopy,  the  refraction  of  the  two  chief 
meridians  in  astigmatism,  the  error  and  correction  are  calcu- 
lated exactly  as  in  any  other  test  for  astigmatism,  and  accord- 
ing to  the  rules  given  in  the  section  on  Astigmatism.  The 
astigmatic  error  is  always  the  difiference — the  algebraical  dif- 
ference— between  the  two  meridians.  Theoretically,  it  mat- 
ters not  whether  this  difference  be  corrected  by  a  plus  or  a 
minus  cylinder.  Having  thus  made  the  two  meridians  alike, 
the  balance  of  the  error,  and  of  the  correction,  is  spherical,  to 
be  treated  accordingly. 

IRREGULAR  ASTIGMATISM. 

The  interpretation  of  this  condition  by  retinoscopy  is  ex- 
ceedingly puzzling.  The  shadow  moves  inconsistently  ;  with 
in  some  meridians,  against  in  others;  and  the  appearance  of 
the  pupillary  area  is  often  a  mixture  of  light  and  darkness. 
In  order  to  refract  such  an  eye  with  the  retinoscope,  the  op- 
erator must  find  the  best  position  of  observation,  and  pick 
out  the  meridians  which  offer  the  clearest  shadow  phenomena. 

SCISSORS  MOVEMENT. 
Occasionally,  in  carrying  out  a  retinoscopic  examination,  the 
shadow,  instead  of  moving  regularly  from  one  side  of  the  eye 
or  the  other,  seems  to  divide  into  two,  and  the  two  halves  ap- 
proach each  other  like  the  blades  of  a  pair  of  scissors,  hence 
the  term,  scissors  movement.  Sometimes  this  phenomenon 
signifies  irregular  astigmatism,  but  more  often  it  is  due  to  a 
tilting  of  the  crystalline  lens  or  a  lack  of  alignment  in  the 
optical  media.  In  such  cases  Sheard  lays  down  two  rules : 
(1)  Be  sure  to  refract  the  eye  along  the  line  of  light;  (2)  If 
scissors  movement  occurs,  watch  carefully  for  the  position  of 


388  RETINOSCOPY 

the  di\isiun  liiu-.  and  refract  that  portion  of  ihc  eye  in  wliich 
the  reflex  contains  within  itself  the  visual  line. 

NEUTRALIZING  WITH  SPHERE  AND  CYLINDER. 

Perhaps  the  simplest  and  most  satisfactory  method  of  all  to 
apply  skiascopy  to  the  astigmatic  eye  is  to  employ  a  combina- 
tion of  spherical  and  cylindrical  power  for  neutralizing  pur- 
poses.   The  technique  is  as  follows : 

Having  placed  the  objective  working  lens  in  position,  and 
holding  the  retinoscope  at  the  focal  length  of  this  lens,  reduce 
the  shadow  with  spherical  neutralizing  lenses  (plus  or. minus, 
as  may  be  indicated)  until  one  meridian  is  neutralized,  and 
the  astigmatic  band  and  other  signs  indicate  the  axis  of  the 
uncorrected  meridian.  Then  apply  cylinders  (plus  or  minus 
as  called  for),  axis  to  coincide  with  the  band,  until  the  other 
meridian  is  neutralized.  The  neutralizing  lenses  in  the  frame, 
aside  from  the  working  lens,  will  then  represent  the  proper  cor- 
rection, without  the  need  of  any  mathematical  calculations. 

DYNAMIC    SKIASCOPY. 

Dynamic  skiascopy  is  a  method  of  retinoscopy  devised  and 
demonstrated  by  A.  fay  Cross,  of  New  York  City,  some  fifteen 
years  ago,  for  measuring  the  refraction  of  the  eye  by  means  of 
the  retinoscope  with  the  ciliary  muscle  in  contraction,  i.  e.. 
with  some  of  the  accommodation  in  force.  Crt)ss  based  his 
wcjrking  theory  upon  two  physiologic  principles:  (1)  That  a 
muscle  in  action  will  accept  assistance  more  readily  than  a 
muscle  at  rest,  and  (2)  That  convergence  and  accommoda- 
tion are  ineluctably  bound  together  in  the  ratio  of  three  to  one 
(i.  e.,  3  prism  dioptres  of  convergence  to  1  dioptre  of  accom- 
modation), which  the  patient  will  luaintain  at  any  fixed  dis- 
tance in  spite  of  anything  that  is  done  to  try  to  break  it. 

'i'he  technicjue  generally  employed  is  as  follows:  A  row  of 
letters  is  attached  t(j  the  toj)  ol  tin-  mirror  of  the  retinoscope. 
which  the  patient  is  re(|uired  to  read  during  the  test;  the  point 
of  fixation  thus  being  the  mirror  itself,  which  is  held  at  any 
convenient  distance  from  the  patient's  eyi'.  (.  ross  avers  that 
it  is  immaterial  whether  «.ik-  rye  be  coxered  or  both  remain 
uncovered  during  the  test,  as  coin  t-rt^ence  will  be  made  tor  the 
point  of  fixation  in  either  case. 


RETINOSCOPY  389 

Let  us  assume  that  the  mirror  is  held  at  50  cm.  working  dis- 
tance from  the  patient's  eye.  As  this  demands  6  prism  dioptres 
of  convergence,  the  patient  will  immediately  put  in  force  2  D. 
of  accommodation  and  maintain  that  amount.  If  he  is  emme- 
tropic, this  2  D.  is  all  the  accommodation  he  is  using.  The 
point  of  fixation  is  then  conjugate  with  the  retina,  and  no 
shadow  is  observable. 

If  the  patient  is  hyperopic  2  D.,  but  has  no  spasm  of  the 
ciliary  muscle,  he  will,  under  the  same  conditions  of  test,  put 
2  D.  of  accommodation  in  force,  but  this  will  not  make  his 
retina  conjugate  with  the  mirror  at  50  cm.,  being,  in  fact,  just 
sufificient  to  make  it  conjugate  with  infinity.  The  emergent 
waves  are  now  neutral,  and  the  shadow  moves  with  the  mirror. 
We  add  plus  1  D.  lens  power,  the  patient  still  maintaining  his 
2  D.  accommodation  ;  we  now  have  plus  3  D.  in  force — 1  D. 
lens  and  2  D.  accommodation — still  not  sufficient  to  focus 
the  emergent  waves  on  the  mirror.  The  shadow  still  moves 
with.  The  addition  of  another  1  D.  will  give  us  4  D.  plus 
power — 2  D.  lens  and  2  D.  accommodation.  This  will  be  suffi- 
cient to  neutralize  the  2  D.  hyperopia  and  focus  the  emergent 
waves  at  the  50  cm.  position  of  the  mirror,  abolishing  the 
shadow. 

If  we  place  any  additional  plus  power  before  the  eye,  the 
patient  will  still  pertinaciously  maintain  his  2  D.  accommoda- 
tion, in  ratio  with  his  convergence,  and  we  shall  have  an  ex- 
cess of  plus  power,  focussing  the  emergent  waves  between  the 
eye  and  the  mirror,  giving  an  "against"  movement  of  the 
shadow.  The  highest  plus  lens  power,  then,  with  which  the 
shadow  can  be  neutralized,  making  no  allowance  for  working 
distance,  is,  according  to  Cross,  the  measure  of  the  hyperopia 
and  its  distance  correction.  And,  if  there  is  no  ciliary  spasm, 
this  finding  will  coincide  with  that  of  the  static  retinoscopic 
test. 

It  must  be  borne  in  mind  that  in  arriving  at  the  finding  of 
the  static  test,  allowance  has  to  be  made  for  working  distance, 
because  the  static  test  is  really  a  test  at  infinity,  which  the 
working  lense  simply  permits  us  to  make  at  a  nearer  point 
for  sake  of  convenience.  In  dynamic  skiametry  the  accommo- 
dation of  the  patient's  eye,  fixed  on  the  mirror,  furnishes  this 


3'X)  RETINOSCOPY 

working-distance  jtowcr,  and  it  does  not.  therefore.  ha\e  to 
be  allowed  for  in  lens  power. 

Suppose,  again,  that  the  patient  is  2  1).  hyperoi)ic.  and  has  a 
ciliary  spasm  of  1  I).  Upon  fixing  the  mirror  at  50  cm.  this 
])atient  makes  2  I).  acti\e  accommodation,  in  accordance  with 
his  convergence-acconinKKlation  relatiKn.  lie  now  has  in  force 
3  D.  plus  power — 1  1).  spasm  and  2  1).  active  accommodation. 
As  it  takes  4  D.  to  make  the  mirror  conjugate  with  his  retina 
(2  D.  to  neutralize  the  hypero])ia  and  2  1).  to  e(|ualize  the  50 
cm.  distance),  he  is  still  short  1  1).  We  place  1  D.  plus  lens 
before  the  eye.  We  now  have  the  total  4  D.  necessary  to 
make  retina  and  mirror  conjugate,  and  the  shadow  is  abolished. 

We  now  go  on  and  add  another  1  1).  plus  lens.  Here  we 
couK'  to  the  ver\'  crux  of  Ooss'  theory.  According  to  his  doc- 
trine, the  patient,  rather  than  break  the  relation  between  his 
convergence  and  his  accommodation,  will  surrender  his  1  D. 
ciliary  spasm,  and  the  conjugate  relation  of  mirror  and  retina 
will  remain  unchanged — there  is  still  no  shadow.  The  addi- 
tion of  another  .50  D.  ])lus  lens.  howe\  er.  will  give  us  an  ex- 
cess of  plus  power,  and  cause  the  shadow  to  mo\e  "against." 

in  this  case  the  highest  plus  power  with  which  we  arc  able 
to  find  a  point  of  re\  t-rsal  is  2  1)..  as  in  the  former  example, 
but  in  this  instance  it  will  not  coincide  with  the  finding 
of  the  static  retinoscopic  test,  which  would  slu)w  but  1  D.  of 
error,  the  other  1  I),  being  locked  in  a  si)asm  at  intinitx.  .\c- 
cording  to  Cross,  therefore,  if  there  be  more  ])lus  lens  in  the 
dynamic  finding  than  in  the  static,  the  difi'erence  represents 
ciliary  spasm. 

There  are  two  objections  to  Cross"  thei)ry,  one  a  hyi)othetical 
objection,  the  other  a  clinical  oiu-.  hirst:  .\ccording  full 
rec(jgnition  to  (he  soundness  ol  the  physiological  principle 
that  a  muscle  in  actit)n  accepts  aid  more  readily  than  a  nuiscle 
at  rest,  yet.  in  \iew  of  (he  pertinacity  of  most  «tf  these  ciliary 
spasms,  it  would  seem  doubtful,  to  >ay  the  least,  whether 
the  acconunodation-con\  ergence  imperati\e  is  snfViciently 
strong  to  force  a  surrender  of  tlu'  sp.ism  in  preference  to 
breaking  the  ihree-tooni'  ratio.  (  W C  s.iy  nothing  .iltout  per- 
maiu-nt  ciliary  contractures.  becan>c  no  theory,  of  course, 
could   contemplati-   the   surrendci    of   tluni    under   te^tJ       This 


RETINOSCOPY  391 

doubt  becomes  still  more  insistent  in  \iew  of  what  we  know 
as  relative  accommodation. 

Second :  Clinical  experience  has  demonstrated,  over  and 
over  again,  that,  where  dynamic  skiametry  has  given  a  larger 
plus  finding  than  static  retinoscopy,  and  correction  accordingly 
given  with  a  view  to  developing  the  latent  error  thus  alleged  to 
be  indicated,  no  latent  error  has  ever  developed.  In  the  early 
days  of  dynamic  skiametry.  when  Cross"  theory  was  accepted 
at  its  face  value,  practitioners  were  in  the  habit  of  giving  full 
distance  correction  in  accordance  with  the  dynamic  finding, 
although  it  fogged  the  patient  badly,  believing  that  in  a  little 
while  the  uncovering  of  the  spasm  would  adjust  the  correction 
to  their  vision.  In  the  vast  majority  of  cases,  however,  this 
happy  outcome  did  not  eventualize ;  patients  came  back  to 
have  their  glasses  changed,  or  they  went  somewhere  else 
and  were  lost  to  the  dynamic  practitioner:  and  so  the  practice 
was  at  length  abandoned. 

The  probabilities,  therefore,  are  strongly  against  the  sound- 
ness of  Cross'  working  theory ;  and  we  understand  that  even 
Mr.  Cross  himself  no  longer  holds  to  his  original  interpreta- 
tion of  his  test,  which,  nevertheless,  remains  a  most  valuable 
procedure.  The  truth  is,  there  is  considerable  difference  of 
opinion  as  to  the  proper  interpretation  to  be  placed  on  the  find- 
ings of  dynamic  skiametry.  even  among  those  who  are  most 
expert  in  its  employment.  Sifting  this  varied  opinion  out.  we 
may,  perhaps,  classify  it  into  three  principal  groups. 

1.  The  followers  of  Cross  still  continue  to  hold  the  general 
view  that  the  dynamic  finding  indicates  the  full  distance  cor- 
rection, but  not  necessarily  the  correction  that  can  or  should 
be  given  the  patient.  It  represents,  in  fact,  the  limit  of  ciliary 
relaxation  in  the  individual  case,  in  much  the  same  way  as  the 
finding  under  atropine  does.  The  objection  to  this  view  is  that 
it  has  Init  little  clinical  value.  All  that  it  affords  the  prac- 
titioner is  a  hint  of  the  probable  (or  possible)  extent  to  which 
the  patient  may  be  expected  to  accept  plus  correction  in  the 
course  of  time — all  of  which,  however,  has  still  to  be  worked 
out  from  time  to  time  if  so  be  that  the  patient  does  develop 
the  need  for  additional  plus  correction. 


392  RETINOSCOPY 

2.  A  more  modern  group  of  refractionists  regard  dynamic 
skiamctry  as  valuable  method  of  demonstrating  and  working 
out  conditions  of  accommodative  inefficiency.  They  make  a 
distinction  between  insufficiency  of  accommodation  and  ac- 
commodative inefficiency.  The  former  is,  of  course,  presby- 
opia, whether  normal  or  ))rcniature  :  the  latter  means  that  the 
ciliary  muscle  is  ind  i)ro|)erl\  innerxated.  Fhe  condition  in 
question  will  be  found  discussed  in  the  chapter  on  Accommo- 
dation. According  to  this  view,  the  excess  plus  finding  of  the 
dynamic  over  the  static  test  represents  the  ciliary  deficiency 
in  that  particular  case  for  the  point  at  which  the  dynamic  test 
is  made.  Under  this  interpretation  the  dynamic  test  is  es- 
sentially a  near-point  test,  and  would  not  be  used  as  the  basis 
of  a  distance  correction.  It  would  be  a  (|uestion  for  the  prac- 
titioner's judgment  whether  he  would  prescribe  reading  cor- 
rection to  help  the  inefficiency  disclosed  by  the  test — the  an- 
swer depending  chieHy  on  the  degree  of  c^-estrain  that  was 
])resent. 

3.  A  third  grou]).  instigated  and  headed  1)\-  Sheard.  is 
probably  nearest  the  truth  in  beliexing  that  the  excess  dif- 
ference between  the  static  and  the  dynamic  finding  rejjresents 
the  negatixe  relati\e  accommodation  of  the  eye.  i.  e.,  tiie 
amount  that  it  can  be  forced  to  surrender  without  changing  the 
convergence.  Not  only  is  such  a  \iew  of  the  matter  the  most 
rational,  but  from  such  a  standpoint  d\namic  skiamctry  as- 
sumes the  highest  practical  value.  It  becomes  an  excellent  meth- 
od of  determining  the  reserxe  accommodrition  ])ower  and  the 
flexibility  <jf  the  acconimodation-coinergence  function;  and 
affords  a  means  of  finding  the  point  at  which  accommodation 
and  con\ergence  coincide. 

Under  this  view,  howexer,  the  technii|ue  of  the  test  is  sonu-- 
what  different  from  the  original  Cross  technii|ue.  To  begin 
witii,  the  full  distance  correction  should  first  be  ascertained, 
and  put  on.  We  will  first  consider  the  application  of  the  test 
to  the  detcnnining  of  the  amount  of  reserx  e  accommodation. 
W  ith  the  patiint's  .iicominodatii  m  fi\(<l  on  the  retinoscope  at 
a  gi\  en  near  ])(»int  say  ,^3  cm.,  w  i-  nuti'  liie  movement  of  the 
shadow.  If  neutral,  we  come  closer  to  the  eye  until  the  motion 
is  "with,"  then   ih-.-iwini-   b.ick   imtil   it   is  neutr.il  ag;iin. 


1 


RETINOSCOPY  393 

The  closest  point  at  which  we  can  still  get  a  neutrality  of 
shadow  is  the  comfortable  near  point.  Converting  this  into 
dioptres  by  dividing  it  into  unity  gives  us  the  maximum  amount 
of  accommodation  that  the  patient  can  use  without  eye-strain, 
and  deducting  this  from  his  total  amplitude  of  accommodation, 
gives  the  exact  reserve  accommodation  required  in  this  par- 
ticular case. 

The  ratio  between  the  total  amplitude  and  the  required  re- 
serve will  determine  the  necessity,  or  otherwise,  of  affording 
the  patient  aid  by  means  of  reading  glasses.  If.  for  example, 
his  total  amplitude  is  5  D.,  his  comfortable  near  point  at  25 
cm.,  which  is  equivalent  to  4  D.  of  accommodation,  showing 
a  required  reserve  of  1  D.,  near  glasses  are  evidently  not 
called  for.  If.  on  the  other  hand,  his  total  amplitude  is  5  D., 
his  comfortable  near  point  the  equivalent  of  1  I).,  showing  a 
required  reserve  of  4  D.,  near  glasses  are  called  for  in  spite  of 
the  25  cm.  near  point. 

\\'hen,  as  sometimes  happens,  in  making  this  test,  the 
shadow  movement  continues  "with"  at  all  convenient  dis- 
tances, extra  plus  lens  power  may  be  placed  before  the  eye, 
the  test  proceeded  with,  and  the  extra  ]>lus  lens  allowed  for  in 
making  the  final  calculation. 

For  the  purpose  of  determining  the  coincidence  of  accom- 
modation and  convergence.  Sheard's  modification  of  the  above 
test  is  applicable.  Full  distance  correction  should  be  worn 
during  the  procedure.  The  patient  is  allowed  to  select  the  dis- 
tance at  which  he  ordinarily  reads,  or  does  the  near  work  for 
which  he  desires  comfortable  vision,  and  the  test  is  made  at 
this  distance.  With  the  accommodation  fixed  on  the  retino- 
scope  at  this  distance,  the  maximum  plus  or  minimum  minus 
lens  is  found  which  just  suffices  to  give  a  neutral  shadow, 
not  carrying  the  correction  to  a  point  where  it  reverses  the 
shadow.  This  lens  power  represents  the  coincidence  of  ac- 
commodation and  convergence. 

Sheard  carries  this  test  a  step  further  for  the  purpose  of 
ascertaining  the  flexibility  of  the  dual  function,  by  trying  out 
l)Oth  the  positi\e  and  negatixe  accommodation  at  the  point  in 
({uestion.  According  to  Donders'  rule,  '"The  accommodation 
can  be  maintained  onlv  for  a  distance  at  which,  in  reference 


394  RETRACTOR 

to  the  negative  part,  tlie  positive  part  of  relative  accommo- 
dation is  toleral)ly  large.  If,  therefore,  the  relative  test  does 
not  fulfill  this  condition — i.  e..  if  the  positive  relative  accom- 
modation is  not  considerably  larger  than  the  negative — then 
that  furnishes  still  further  ground  for  supplying  the  patient 
with  reading  assistance,  as  indicated  by  the  dynamic  test. 

Sheard.  therefore,  believes  tliat  dynamic  skiametry,  when 
properly  practiced,  affords  a  cpiick  and  accurate  method  of 
finding  the  lens  assistance  needed  at  reading  distance  to  re- 
establish the  normal  relations  between  the  positive  and.  nega- 
tive ranges  of  accommodation  and  the  convergence. 

THE   NEAR   POINT   BY    SHEARD'S    METHOD. 

Sheard  further  holds  that  the  near  poir.t  cannot  be  accu- 
rately found  by  dynamic  skiametry  as  it  is  usually  practiced, 
i'-or  this  purpose  he  uses  the  following  techni(iue: 

The  fi.\ati(m  cliart  is  pushed  in  ad\ance  of  the  retinoscope. 
With  full  distance  correction  on.  the  patient's  attention  is  di- 
rected to  the  chart  held  two  or  three  inches  in  front  of  the 
mirror,  and  the  operator  notes  the  movement  of  the  shadow. 
If  it  is  "with"  he  ])ushes  the  chart  forward  until  it  becomes 
"against."  He  then  comes  forward  with  the  mirror  until  it  is 
"with''  again;  and  so  on.  until,  with  the  chart  some  distance 
in  front  of  the  mirror,  he  locates  the  nearest  point  of  neutral- 
ity, which  denotes  the  true  near  point  of  the  patient. 

The  working  princii)k'  of  this  procedure  is  lliat  the  patient 
must  fix  a  point  well  within  his  near  point,  the  observer  com- 
ing up  from  behind  to  locate  the  neutral  shadow,  which  must 
be  the  true  punctum  proximum. 

Retractor.  An  instrunu-iU  for  holding  back,  or  ojjcn.  parts  of 
the  body  during  oi)eration.  as  lid  retractors. 

Retrobulbar  Neuritis,     .'^cr  Neuritis.  Optic. 

Reversal.  in  optics  this  term  i>  specilically  applied  to  the 
changing  of  a  light-wave  from  one  of  positive  cur\  ature  to  one 
of  negative  cnr\aturc.  or  the  oi)posite. 

Point  of  Ke\ersal.  In  the  changing  of  a  negative,  or  iniiui> 
wave  into  a  positive,  or  )>lus  one.  it  is  necessary  that  tin-  minus 
wave  be  brought  to  ;i  focus,  from  which  it  st.irts  to  expand 
into  a   poNitixe   wa\e.       This   focal   point   where  tlii'   change  oc- 


RHYTIDOSIS  395 

curs  is  technically  known  as  the  point  of  re\'ersal.  The  term 
is  specifically  applied  to  the  reversal  of  the  emergent  waves 
from  the  eye  in  retinoscopy. 

Rhytidosis.     Wrinkling. 

Right-Eyed.     Ha\ing'  the  right  eye  the  dominant  eye. 

Robertson,  Argyll,  Pupil.     See  Argyll  Robertson  Pupil. 

Rods  and  Cones.     See  Retina. 

Rotation.  A  method  of  determining"  the  the  polarization  of  light 
(See  Polarization). 

Rotation  of  the  mirror  is  the  technical  term  describing  the 
tilting  of  the  mirror  on  its  stem  axis  which  is  used  in  the 
technique  of  retinoscopy.      (See  Retinoscopy.) 

Sarcoma.  A  form  of  malignant  tumor  characterized  by  large- 
cell  elaboration  of  embryonic  tissue.  It  sometimes  attacks  the 
orbit  of  the  eye. 

Schematic  Eye.  A  model  eye,  built  on  the  same  plan  as  the 
human,  for  purposes  of  study  and  experiment. 


Schematic  Eye. 


3%  SCHLEMM'S   CANAL 

Schlemm's  Canal.     St,c  Canal. 

Scissors  Movement.  A  moxtiiK'nt  of  the  shadow  in  rctinoscopy 
rescnihling  tiie  moxement  of  a  j)air  of  scissors,  i.  c.  instead 
of  the  two  sides  <»f  the  shadow  nio\in<j^  laterally  and  conju- 
gately.  they  move  toward  and  away  from  each  other.  It  in- 
dicates irregular  astigmatism.      (See  Retinoscopy. ) 

Sclera.  Sclerotic.  I'he  outer  coat  of  the  eyehall,  extending  from 
the  optic  ner\e  forward  to  the  insertion  of  the  cornea.  It  is 
firm  and  white,  and  relati\ely  thick,  especially  at  the  back. 
where  it  has  a  thickness  of  about  1  mm.  It  gives  shape  and 
supjiort  to  the  globe,  and  ser\  es  as  the  dark  box  of  the  camera 
of  the  eye.  It  is  composed  of  connective  tissue,  and  to  it  are 
attached  the  extrinsic  muscles. 

Scleractasis.      P>ulging  of  the  sclera. 

Scleriritomy.  Incision  oi  the  sclera  and  iris  fur  the  relief  of 
anteri(jr  staphyloma. 

Sclerochoroiditis.  Joint  intlanimation  of  ihe  sclera  and  the 
choroid. 

Sclero-Conjunctival.  Pertaining  lo  the  line  uf  junction  betwren 
the  sclera  and  conjunctiwi. 

Sclero-Corneal.  Relating  to  the  margin  of  junction  ln-tween  the 
sclera  and  the  cornea. 

.Sclero-corneal  .Sulcus.  The  de])ression  formed  in  the  sclen*- 
corneal  margin  by  the  ditT(  innce  of  cur\  atun-  of  the  sclera  and 
cornea,  respectix  ely. 

Sclero-Iritis.      Iiitlammation  of  the  sclera  and  iris. 

Sclero-Kerato-Iritis.  Inllamniation  of  the  sclera,  cornea  and  iris 
--iniultaneouslw 

Scleronyxis.      Perforation  of  the  scK-ra. 

Sclerophthalmia.  A  condition  in  which  the  scleia  encroaches  on 
the  cornea,  so  that  only  lln'  central  portion  of  tlu'  latter  is 
transparent. 


i 


SCLEROSED  3*^7 

Sclerosed.     Hardened. 

Sclerotic.     See  Sclera. 

Scleroticectomy.     Cutting  of  the  sclera. 

Scopolamine.  An  alkaloid,  identical  with  hyoscine,  which,  when 
instilled  in  the  eye,  produces  midriasis.  It  is  sometimes  em- 
ployed in  preference  to  homatropine.     Its  action  is  transitory. 

Scotodinia.     Dizziness  and  dimness  of  vision. 

Scotoma.  A  dark  spot  in  the  visual  field,  due  to  a  corresponding 
blind  spot  on  the  retina.  When  the  retinal  area  is  totally 
blind  the  scotoma  is  called  absolute.     See  Perimetry. 

Scotom,eter.  An  instrument  for  determining  the  position  and 
extent  in  the  visual  field  of  scotomata,  from  which  data  one 
is  able  to  locate  the  retinal  lesion  which  is  responsible  for  them. 

Sebaceous  Cyst.  Infection  and  swelling  of  one  of  the  sebaceous 
glands  of  the  eye. 

Second  Sight.  In  the  early  stages  of  certain  cataracts,  the  crys- 
talline lens  swells  without  undergoing,  as  yet,  much  opacity. 
This  swelling  of  the  lens  increases  its  curvature,  thus  ofifsetting 
a  good  deal  of  the  patient's  presbyopia,  and  as  a  consequence 
these  patients  are  often  able  to  read,  for  a  period,  without 
their  glasses.  This  is  known  as  second  sight.  The  patient, 
ignorant  of  the  cause,  is  usually  very  proud  of  the  feat ;  but 
to  the  ophthalmologist,  of  course,  it  is  a  serious  indication  of 
approaching  trouble. 

Segment.  A  piece  cut  out  of  a  circle  or  sphere.  All  lenses  are 
segments  of  a  sphere  or  of  a  cylinder. 

Senopia.     Second  sight,  q.  v. 

Serpiginous.  A  term  applied  to  a  certain  type  of  corneal  ulcer 
which  creeps  rapidly  over  the  surface  in  serpentine  form.  It 
is  a  very  grave  condition. 

Shadow  Test.     Same  as  retinoscopy. 


308  SHEATH 

Sheath.  A  tubular  case  or  cosering.  All  large  nerves  are  fur- 
nished with  sheaths,  for  insulation  purposes.  The  sheath  of 
the  optic  ncr\  c  is  an  extension  of  the  dura  mater  of  the  l)raiii. 

Short-sightedness.     A  collotpiial  term  for  myopia. 

Sideroscope.  A  magnetic  instrument  for  determining  the  pres- 
ence of  a  piece  of  metal  in  the  eye. 

Siderosis.  .\  characteristic  condition,  including  a  rusty-brown 
discoloration,  which  is  seen  in  the  eye  as  the  result  of  the  long- 
continued  ])resence  of  a  piece  of  metal  in  it. 

Sine.  A  term  in  trigonometry,  to  express  curve  measurement  in 
linear  fashion.  The  sine  of  an  arc  is  a  line  drawn  from  one 
end  of  the  arc  perpendicularl}'  upon  the  diameter  drawn 
through  the  other  end  of  the  arc.  The  sine  of  the  arc  is  also 
the  sine  of  the   angle  subtending   the  arc. 

The  relati\"e  \alues  of  the  sines  of  the  angles  of  incidence 
and  refraction  constitute  the  index  of  refraction. 

Skiascope.     Same  as  Retinoscope,  ([.  v. 

Skiascopy.     .Same  as  Retinoscopy,  (|.  \'. 

Snow-Blindness.  Ivxhaustion  of  the  rods  and  cones  of  the  retina, 
due  to  long  exposure  to  the  white  light  rellected  fnun  snow. 

Socket.  The  cone-like  hollow  formed  by  the  bones  of  the  orbit 
into  which  the  eyeball  is  set. 

Spectacles.  .Stricth  speaking,  the  word  spectacles  refers  to  the 
lenses  wdiich  are  adjusted  to  the  human  eye  to  correct  defects 
fjf  \ision,  the  containing  parts  l)iing  known,  in  the  case  of 
bow-s])ectacles,  as  the  frame,  .md  in  the  case  of  eye-glasses  as 
mountings.  Recent  usage.  howe\er.  has  tended  to  appl\  the 
term  to  the  mechanical  parts,  rather  than  to  the  lenses:  and 
it  will  be  so  considered  ;iud  detined  here. 

Spectacles  and  eye-glasses  ;ire  made  eitlu-r  with  or  without 
rims,  or,  as  the  opticians  c;dl  iluni,  eye-w  ires.  In  the  limnu'd 
variety  a  p.iir  of  spectacle  Ir.nnes  consists  of  the  eye-wires, 
end-pieces,  tem|)les  .ind  bridge.     '\'\\v  end-pieces,  ftuir  in  nnm- 


SPECTACLES  399 

ber,  are  attached  in  pairs  to  the  outer  sides  of  the  eye-wires; 
one  end-piece  is  furnished  with  a  projecting  pin,  or  dowel, 
which  fits  into  the  receiving  hole  on  the  opposite  piece. 
Both  pieces  are  pierced  with  a  threaded  hole  for  the  screw. 
The  temple  is  flattened  at  its  near  end  and  pierced  with  a 
hole  corresponding  to  the  pin  on  the  end-piece.  The  temple 
is  placed  between  the  two  end-pieces,  the  pin  passing  through 
the  hole  in  the  temple  into  the  receiving  hole  in  the  opposite 
end-piece.  A  screw  is  then  driven  through  the  threaded  hole 
in  the  end-pieces,  so  as  to  hold  the  lens  in  place  in  the  tautened 
eye-wire  and  the  temple  between  the  end-pieces.  The  temples 
revolve  on  the  pin  inward,  but  are  prevented  from  revolving 
outward  by  a  small  flange  on  the  temple.  The  bridge  is 
soldered  to  the  inner  sides  of  the  eye-wires. 

In  rimless  spectacles  the  general  arrangement  is  the  same, 
but  the  end-pieces,  and  the  bridge,  are  attached  to  the  lenses 
themselves  by  means  of  small  clutches  which  are  screwed  onto 
the  lenses  through  holes  drilled  in  the  glass. 

Of  late  years  the  metal  eye-wires  of  rimmed  glasses  have 
been  replaced  by  rims  made  of  various  kinds  of  shell.  These 
rims  are  shrunk  onto  the  edge  of  the  lens  by  first  heating 
them  and  then  allowing  them  to  cool  in  place,  just  as  metal 
tires  are  shrunk  onto  a  wheel.  The  bridge  is  attached  to  the 
shell  rim  in  various  ways. 

TEMPLES. 

Fundamentally,  the  only  two  varieties  of  temple  are  those 
which  curl  around  the  back  of  the  ear  and  those  which  do  not, 
the  only  practical  difference  being  one  of  comfort  and  stability. 
The  former  "hook"  or  ''riding  bows"  are  better  for  constant 
wear,  the  latter,  or  "straight,"  for  patients  who  have  to  be  con- 
tinually taking  them  ofT  and  putting  them  on.  Some  of  the 
riding  bows  are  made  of  flexible  material  and  are  known  as 
"cable  riding  bows,"  in  distinction  from  the  firm  variety, 
which  are  called  "curls."  Temples  are  also  made,  nowadays, 
of  shell. 

BRIDGES. 

As  the  bridge  is  the  part  whose  shape  and  position  regu- 
lates the  lateral  and  vertical  relation  of  the  lenses  to  the  eyes. 


4(X)  SPECTACLES 

its  adjustment  is  a  matter  of  great  imixjrtance.  There  arc 
innumerable  \  arieties  of  bridges,  which  it  would  be  folly  to 
attempt  t(j  enumerate,  far  less  describe,  here.  They  may  be 
found  in  the  catalogs  of  various  optical  houses.  'Jhey  fall, 
however.  like  temples,  into  two  fundamental  classes,  namely, 
the  wire  and  the  saddle  bridge.  The  former  are  intended  to 
be  shaped  to  the  nose  and  are  adaptable  to  persons  having 
well-shaped  noses;  but  for  the  a\  erage  nose  a  saddle  bridge, 
made  in  the  shape  of  a  letter  K.  wliich  may  be  as  flat  or  as 
deep  as  desired.  ser\  cs  the  purpose  best. 

EYE-GLASSES. 

In  eye-glasses,  of  course,  there  are  no  temples.  On  the  right 
outer  side  of  the  eye-wire  in  rimmed  e>e-glasses.  of  the  lens 
itself  in  rimless  ones,  the  end-piece  is  enlarged  into  a  post  and 
handle  with  which  to  manipulate  the  glasses.  To  the  inner 
sides  of  the  rims  or  of  the  glasses  are  attached  the  nose-piece, 
which  is  usually  furnished  with  a  guard  of  some  resilient  ma- 
terial to  keep  it  from  abrading  the  nose. 

SPRING   AND   NOSE   PIECE. 

In  eye-glasses  the  bridge  of  the  spectacle  is  replaced  by  the 
spring  and  the  nose-piccc.  Like  the  bridges  of  spectacles, 
their  \'arietv  is  legion,  and  ilicy  obey  the  same  principles  as 
the  bridge,  namely,  in  determining  the  position,  plane,  and 
distance  from  the  eyes,  of  the  lenses,  according  to  the  dif- 
ferent angle  at  which  the  guards  are  attached  to  the  lenses 
or  rims  and  the  distance  aboxe  or  below  center  at  which  the 
studs  are  attachetl  to  the  lenses. 

STUDS. 

In  eye-glasses,  as  in  spect;icles,  it  is  ofti'ii  necessary  to  set 
the  lenses  forward  or  batkward,  .accordnij;  to  the  prominence 
or  shallowness  of  the  nose  and  eyes.  I  his  is  done  by  means 
of  special  studs,  known  as  inset  and  outset  studs. 

Spectrum.  The  dirferrnti.-ition  of  tlu-  white  pencil  of  li^lu  into 
its  constituteut   cohir   wa\i'S.     l^ee  Color. 

Sphenoid.  <  >nc  oi  the  bones  of  the  oibil.  It  is  ;i  peculiarly- 
sliaped  bone,  somewhat  like  ;i  liat  willi  txtcinU-tl  wini^s.  and 
articulates   with   e\erv  other   bone   in   the  oibil. 


SPHERE  401 

Sphere.  This  word  is  used  to  designate,  shortly,  a  lens  whose 
surface  is  a  segment  of  a  sphere.     See  Lens. 

Spherometer.  An  instrument  for  measuring  the  curvature  of  a 
sphere. 

Sphincter.  A  ring-shaped  muscle  which  contracts  concentrically. 
The  eye  has  two  such  muscles,  the  sphincter  iridis  and  the 
ciliary. 

Spintherism.  A  condition  in  which  the  patient  sees  stellar  flashes 
of  light. 

Squint.    See  Strabismus. 

Staphyloma.  A  cone-like  bulging  of  one  of  the  tissues  of  the 
eye.  An  anterior  staphyloma  is  a  bulging  forward  of  the  cor- 
nea ;  a  posterior  staphyloma  is  a  bulging  backward  of  the 
sclera  and  choroid.  In  either  case,  of  course,  it  lengthens  the 
eyeball,  and  tends  to  create  a  condition  of  myopia.  It  is  there- 
fore associated  with  myopia,  especially  high  m}opia.  It  also 
renders  thin  the  tissue  that  is  bulged,  by  stretching  it. 

Static.  In  a  state  of  rest.  Static  refraction  is  measurement  of 
the"  dioptrism  of  the  eye  while  the  accommodation  is  not  in 
force. 

Stenopaic  Slit.  An  opaque  disc  with  a  linear  slit  in  it,  which, 
when  placed  before  the  eye,  permits  light  to  enter  it  along  one 
meridian  only.  It  is  used  for  detecting  and  measuring  astig- 
matism.    (See  Astigmatism.) 

Stereoscope.  Under  this  name  are  included  two  types  of  optical 
instruments,  (1)  those  which  are  designed  to  assist  and  en- 
large, and  in  some  instances  even  to  replace,  the  stereoscopic 
faculties  of  the  eye.  and  (2)  those  which  present  plane  pic- 
tures to  the  eyes  in  such  a  way  as  to  create  the  semblance  of 
depth  and  solidity.  All  binocular  optical  instruments,  in- 
cluding spectacles,  and  certain  specially  constructed  appara- 
tuses, come  within  the  first  class.  It  is  the  second  class,  how- 
ever, that  we  usually  understand  by  the  term  stereoscope. 


402  STEREOSCOPIC  VISION 

The  prime  cleniciil  in  stcreoptic  \  isiun  is  the  parallax,  i.  e.. 
the  different  \  iew  of  the  object  experienced  by  the  two  retinae 
by  reason  of  \ie\ving  it  from  different  angles.  This  is  the  prin- 
cii)le  of  the  stereoscojje.  It  enaldes  us  to  look  simultaneously 
at  two  photographic  pictures,  taken  under  a  difference  of 
angular  \iew,  corresponding  lo.  or  e\en  greater  than,  the 
separation  of  the  two  e}es,  and  thus,  as  in  ordinary  \ision.  the 
two  images  are  fused  by  the  brain,  and  the  objects  thus  viewed 
are  made  to  stand  out  in  relief. 

Wheatstone  invented  a  reflecting  stereoscoj^e.  compounded 
of  mirrors,  in  1839,  which  never  attained  any  worthwhile  re- 
sults, partly  because  of  its  unwieldy  shape,  and  partly  because 
of  the  impossibility  of  obtaining  equal  illumination  on  the 
two  pictures.  Later,  Sir  Da\'id  Brewster  invented  a  refracting 
stereoscope,  made  with  prisms,  upon  which  all  the  modern 
stereoscope  now  in  use  are  modeled. 

Stereoscopic  Vision.  binocular  \  ision  in  which  solid  objects 
stand  out  in  solid  form,  or.  to  use  the  technical  expression,  in 
relief,  and  not  as  Hat  pictures,  h^or  a  detailed  description  of 
this  faculty  see  Binocular  Vision. 

Stillicidium.     ()\erllo\v  of  tears  due  to  stricture  of  the  tlucts. 

Stilling's  Canal.     See  Canal. 

Stilus.     An  instrument  used  for  tiilating  the  tear  tlucts. 

Stop-Needle.  A  needle  used  for  piercing  the  cornea,  with  a 
shoulder  to  limit  the  depth  of  penetration.  The  most  com- 
mcMily  used  is  the  I'.owmaii  stop-needle. 

Strabismus.  A  state  oi  i)ennanenl  dexiation  ol  one  or  both 
visual  axes  from  parallelism.  It  may  occur  as  the  result  of 
paralysis  of  one  of  the  ocidar  muscles  (p.iralytic  strabismus), 
or  as  the  result  of  muscular  imbalance  due  to  error  or  refrac- 
tion (ccjncomitant  strabismus).  In  addition,  there  must  be 
mentioned  a  condition  known  as  apparent  strabismus,  due  to 
an  exceedingly  w  idc  .in^le  alph.i  in  hy|»eidpia  or  an  exceetlingly 
narrow  one  in  myopia,  whicli  j^ives  the  eyes  the  appearance  of 
turning  outward  ami  inwaid,  ri-specti\  el\ .      This  condition  is 


STRASBISMUS  403 

easily  distinguished  from  real  strabismus  by  demonstrating  the 
absence  of  diplopia  under  proper  tests. 

DIFFERENTIATING  REAL  STRABISMUS. 

Since  strabismus  is  in  reality  a  permanently  manifested  im- 
l')alance,  it  is  subject  to  the  same  principles  and  methods  of 
test  as  heterophoria.  Thus,  the  distinction  between  paralytic 
and  concomitant  strabismus  rests  upon  the  same  principle,  and 
is  made  in  the  same  way.  as  that  between  anatomic  and  func- 
tional imbalance. 

(a)  If  the  deviating  eye  be  made  to  follow  a  small  object 
moved  laterally  before  the  eye,  it  will  be  seen  to  lag.  or  to 
stop  entirely,  in  the  field  of  movement  of  the  affected  muscle, 
wheras  in  concomitant  squint  there  will  be  little  or  no  limita- 
tion of  these  conjugate  movements. 

(b)  The  patient  is  made  to  fix  a  point  at  infinity,  first  with 
the  sound  eye,  the  affected  eye  being  covered,  but  so  that  the 
operator  can  observe  it,  then  with  the  bad  eye,  the  sound  eye 
being  covered  in  like  manner.  The  amount  of  deviation  made 
by  the  covered  eye  from  the  median  in  each  case  is  measured 
by  means  of  a  pair  of  calipers  :  that  which  is  made  when  the 
sound  eye  fixes  being  known  as  pruwary  deviation,  and  when 
the  bad  eye  fixes  as  secondary  deviation.  In  paralytic  stra- 
bismus secondary  de\  iation  is  greater  than  primary,  because 
of  the  exaggerated  innervation  required  to  fix  with  the  paralytic 
eye.  In  concomitant  squint  primary  and  secondary  deviation 
are  equal. 

Paralytic  squint  is  usually  the  result  of  an  infectious  disease, 
such  as  diphtheria,  scarlatina,  etc.,  which  poisons  the  periph- 
eral nerve  supplying  the  ocular  muscle,  or  else  of  a  central 
ner\e  lesion  in  the  brain,  due  to  syphilis,  tuberculosis,  menin- 
gitis, brain  tumor,  etc. 

CONCOMITANT   STRABISMUS. 

This  name  is  given  to  functional,  refractive  strabismus  be- 
cause, as  stated,  ni  conjugate  excursions  the  two  eyes  keep 
pace  with  each  other.  Only  in  the  performance  of  conver- 
gence, positive  or  negative,  are  the  excursions  of  the  two  eye.^ 
unequal.  This  type  of  strabismus  is  the  result  of  refractive 
error. 


404  STRASBISMUS 

Internal  concomitant  straUisnuis,  wlu-rc  the  deviating  eye  or 
eyes  turn  inward,  is  usually  the  outcome  oi  hyperopia,  where 
the  patient  has  g^ix  en  up  tryinj:^  to  maintain  fusion  under  the 
strain  of  esophoria,  and  one  or  both  eyes  are  ])ermitted  to  take 
up  a  permanent  position  of  stable  ec|uilibrium.  with  the  visual 
axes  converging',  lliere  is  diplojiia,  of  course,  but  the  patient 
— especially  if  it  be  a  child — finds  it  easier  to  disregard  the 
false  image  in  the  deviating  eye  than  to  endure  the  annoyance 
of  maintaining  unstable  e(|uilibrium  in  the  muscles.  In  some 
cases  the  patient  uses  one  eye  exclusivel}'.  all  the  time,  allowing 
the  other  to  dexiate  continuously,  and  disregarding  its  image  : 
in  which  e\ent  the  attentixe  visual  faculty  of  the  unused  eye 
is  graduall}'  lost,  and  it  becomes  amblyopic  (amblyopia  ex 
anopsia).  In  other  cases,  the  patient  uses  first  one  eye  and 
then  the  other,  allowing  the  other  to  deviate  alternately  (al- 
ternating strabismus)  :  and  in  this  form  of  s(piint  the  \isual 
acuity  of  both  eyes  is  retained  entire. 

In  the  rather  rare  cases  where  both  eyes  are  i)ermitte(l  to 
deviate,  it  is  extremely  difficult  for  the  patient  to  fix  at  in- 
finity except  when  one  eye  is  covered. 

External  concomitant  strabismus,  where  the  de\iating  eye 
turns  outward,  cannot  so  uniformly  or  definitely  be  ascribed 
to  myojua,  although  it  usually  occurs  in  myopic  j)eople.  Such 
a  \ariety  of  scpiint  is.  in  fact,  extremely  rare,  as  explained  un- 
der lieterophoria.  Most  cases  of  external  strabisnuis  are  paral- 
ytic, or,  at  least,  anatomic,  in  their  origin. 

There  is  no  paralysis  of  the  muscle  in  concomitant  scjuint, 
at  least  at  first,  although  long  continuation  may  develop  in 
the  deviating  eye  a  partial  ])aralysis  from  disuse.  In  alternat- 
ing s(piint.  of  course,  this  does  not  occur. 

DEGREES   OF    STRABISMUS. 

.Strabismus  is  measureil  in  the  same  way  as  imbalance, 
namely,  by  the  amount  ol  prism  power  required  to  fuse  tiie 
images.  .\o  dissociation  is  needed,  ot  course,  the  purpose  o\ 
diss(jciati(ni  being  to  turn  an  imbalance  into  a  S(piint.  and  here 
the  transformation  has  already  been  done  by  the  patient. 

The  base  of  the  measuring  ])rism  is  always  to  be  placed  o\  er 
the  "weak"  muscli',  i,  e.,  base  out  when  measuring  inteiiial 
strabisnuis  and  b;ise  in   wluii   nu'asurini;  i-xternal   sijuint. 


STRASBISMUS  405 

There  are  various  mechanical  methods  and  devices  tor  meas- 
uring the  amount  of  deviation  in  a  squinting  eye.  .\  rather 
crude  method  is  by  means  of  the  strabismometer,  consisting  of 
a  handle  supporting  a  small  ivor}-  plate  shaped  to  the  lower 
eyelid,  and  having  on  it  a  millimeter  scale  by  which  the  amount 
of  deviation  can  be  read  off  while  the  sound  eye  is  fixing  at 
infinity. 

A  better  method  is  to  measure  the  angle  of  deviation  by 
means  of  a  perimeter,  consisting"  of  a  central  rest  for  the  pa- 
tient's face  and  a  hemispherical  quadrant  on  a  level  with  the 
patient's  eyes.  With  the  patient  seated  at  the  center,  with 
his  eyes  fixing  infinity,  the  operator  moves  a  lighted  candle, 
or  tiny  electric  bulb,  slowly  along  the  inside  of  the  quadrant, 
following  it  with  his  own  head,  until  it  reaches  the  place  where 
the  reflected  image  of  the  light  is  exactly  in  the  center  of  the 
patient's  deviating  pupil.  A  scale  on  the  quadrant  shows  the 
angular  deviation. 

TREATMENT   OF    STRABISMUS. 

The  treatment  of  paralytic  strabismus  reall}'  belongs  in  the 
province  of  the  medical  man,  and  consists  principally  in  treat- 
ing the  constitutional  condition  underlying  the  paralysis.  How- 
ever, proper  correction  of  refractive  errors,  if  any  exist,  and 
intelligently  directed  muscle  exercises  are  of  considerable  aid. 
(See  Muscle  Exercises.)  Where  these  measures  fail,  tenotomy 
or  advancement  may  be  performed  for  esthetic  effects. 

The  treatment  of  functional  strabismus,  like  that  of  hetero- 
phoria,  is  a  matter  on  which  no  dogmatizing  is  possible.  In 
each  individual  case  the  refractionist  must  use  his  best  judg- 
ment. 

In  arriving  at  a  decision,  we  usually  recognize  three  degrees 
of  severity:  (1)  Where  the  reflected  image  of  a  candle-flame, 
held  about  a  meter  from  the  deviating  eye,  in  the  median  line, 
falls  within  the  pupillary  area,  (2)  Where  this  image  falls  out- 
side the  pupil  but  within  the  cornea,  and  (3)  W'here  it  falls 
outside  the  cornea,  i.  e.,  there  is  no  corneal  rejection.  In 
squint  of  the  first  degree,  it  is  almost  certain  that  proper 
glasses  and  muscle  exercises  will  correct  it,  provided  there  is 
good  vision  in  the  squinting  eye.     In  squints  of  the  second  de- 


406  STRABOTOMY 

gree.  there  is  a  balance  of  possibility  of  success  and  failure, 
only  to  be  decided  by  trying.  In  third  degree  squints  it  is  al- 
most certain  that  an  operation  will  have  to  precede  other  means 
of  cure. 

Needless  to  say  that  accurate  refraction  is  the  first  sine  qua 
non  of  treatment.  In  many  cases  of  esotropia,  due  to  hyper- 
opia, especially  in  young  children,  this  is  suflficient ;  under  the 
correction,  the  eyes  gradually  resume  their  normal  state.  In 
others  the  correction  has  to  be  supplemented  by  properly 
arranged  muscle  and  prism  exercises  designed  to  re-educate 
the  patient  in  the  faculty  of  fusion.  For  a  full  discussion  of 
this  procedure  see  Muscle  Exercises. 

The  most  difficult  problem  is  presented  in  cases  where  the 
deviating  eye  has  become  amld\opic.  Here,  both  correction 
of  refraction  (so  far  as  the  amblyojiic  eye  is  concerned)  and 
education  in  fusion  are  impossible.  Attention  must  first  be 
directed  toward  restoring  visual  function  to  the  bad  eye  until 
it  reaches  a  point  where  it  can  be  induced  to  do  team  work 
with  the  good  one.  A  simple  method  of  doing  this  is  to 
instruct  the  patient  to  cover  the  good  eye  with  a  bandage  or 
patch  for  several  hours  a  day.  and  force  the  use  of  the  ambly- 
opic eve.  .\  more  scientific  and  rapid  method  is  by  the  use  of 
the  amblyscope.  which  educates  the  \isual  acuity  of  the  eye 
and  the  muscle  sense  at  the  same  time.  For  technicpie  of  its 
use  the  reader  is  referred  to  the  section  on  Amblyscope. 

Strabotomy.      1  he  same  as  tenf)tomy. 

Stye.     See  Hordeolum. 

Subconjunctival.  Indenu-alh  the  conjujictixa,  between  it  and 
the  scli-ra. 

Subjective.  In  physiology  tlii>  urni  dciKiiis  jtluiioincna  which 
occiu'  as  sensations  and  fxpcricnccs  within  ilu-  individual's 
own   consciousness. 

In  optometry,  subjectixt-  ti-sls  arc  tiuiM-  which  di'i)<.n(l  for 
their  findings  upon  what  llic  patiint  ti-IN  ns  of  his  own  feelings 
and  percei»tions.      I  lie  princip.il  subjert i\  e  lest  is  tli.il   ol   read- 


SUBLATIO  RETINAE  407 

ing  the  letters  on  the  chart,  and  discerning  the  spokes  of  the 
astigmatic  Avheel. 

Sublatio  Retinae.     Detachment  of  the  retina. 

Subluxation.  A  partial  displacement  of  the  lens,  by  which  it  is 
slightly  tilted.  It  manifests  itself  by  an  ine(|uality  in  the 
depth  of  the  anterior  chamber  of  the  eye.  Its  diagnosis  and 
treatment  belong  to  medicine. 

Suborbital.     Beneath  the  orbit. 

Subretinal.     Underneath  the  retina,  between  it  and  the  choroid. 

Subvolution.     An  operation  for  i)terygium,  in  which  the  growth, 

after  being  split  and  cut,  is  buried  beneath  the  conjunctiva. 
Superciliary.    Pertaining  to  the  eyebrows. 

Supraorbital.     Above  the  orbit. 

Sursumduction.  The  turning  of  one  of  the  eyes  upward  by  the 
superior  rectus  muscle — which  also  implies  the  turning  of  the 
other  eye  downward  by  the  inferior  rectus. 

Sursumduction  is  tested  b}-  the  amount  of  prism,  base  up  or 
base  down,  as  the  case  may  be,  which  the  eyes  are  able  to 
overcome  and  still  maintain  single  vision. 

Suspensory  Ligament.  A  circle  of  delicate,  homogeneous  fibres, 
taking  their  rise  from  the  inner  surface  of  the  ciliary  body, 
close  to  the  ora  serrata,  which  surround  the  crystalline  lens 
and  hold  it  it  place.     It  is  also  called  the  zonula  ciliaris. 

Symblepharon,.     Sticking,  or  growing  together  of  the  eyelids. 

Sympathetic  Ophthalmitis.  An  inflammation  of  one  of  the  eye- 
balls due  to  injury  or  disease  of  the  other  eye.  The  communi- 
cation takes  place  through  the  consensual  fibres  of  the  optic 
nerve  and  the  sympathetic  ganglia  of  the  fifth  nerve. 

Synchysis.  Licjueification  of  the  \itreous.  It  is  made  manifest 
by  the  floating  about  of  opacities  in  the  humor.  It  may  occur 
as  one  of  the  ordinary  events  of  old  age  :  but  in  younger  life  is 
a  sign  of  disease  of  one  of  the  adjacent  tissues. 


408  SYNECHIA 

Synechia.  Adhesion  of  the  iris  to  the  lens  (posterior  synechia) 
or  to  the  cornea  (anterior  synechia),  due  to  exudative  inflam- 
mation of  the  iris,  in  which  sticky  exu(hites  are  poured  out 
which  gkie  the  iris  to  one  of  these  bodies.  It  is  to  i)revent 
synechiae  that  the  ocuh'st  uses  atropin  in  diseases  of  the  iris 
and  cornea,  in  order  to  draw  the  iris  up  out  of  reach  of  the 
lens  and  cornea. 

Synizesis.     Contraction  of  the  pupil. 

Synophthalmus.     A  one-eyed  monster. 

Syntropic.  A  condition  in  which  the  eyes  are  both  turned  in  one 
direction.     Conjugate  dexiation  of  the  e^'es. 

System.  In  (Ji)tics  the  word  denotes  a  combination  of  lenses  or 
mirrors,  in  series,  whose  joint  action  jiroduccs  the  optical  ef- 
fect desired. 

T.     An  al)brc\iation  for  tlu'  inlra-ocular  tension. 

Tabes  Dorsalis.  Another  name  for  locomott)r  ataxia.  One  of  its 
earliest  symptoms  is  the  -Argyll  Robertson  pupil,  i.  e.  a  pupil 
which  does  not  contract  in  response  to  light  stimulus,  but 
contracts  during  accommodation.  It  is  also  fre(iuentl\  accom- 
l)anied  by  o])tic  atrophy. 

Talbot's  Law.     A  law   of  illuminatitm.     If  a  represents  the  dura- 

ti(jn  of  the  llasli,  b  tbr  duration  of  the  eclipse,  then  a/a  —  b  = 
the  intensity  of  the  illumination  gi\i-n  by  tlu'  tlash. 

Tangent.  A  lim-  wbuli  touches  the  circumference  of  a  circle  or 
sphere,  but.  being  ]>ro<lnced,  does  not  cut  it.  The  normal  ol  a 
circle  (M"  sphere  is  i)erpendicular  .also  to  the  tangent  at  the 
point  where  the  latter  touches  the  circumference. 

Tapetum.  I  be  pupillai\  rellex  of  lowi-r  aniniaU.  which,  on 
.account  of  tlie  differenci'  bi'twceu  tlu'ir  refraction  and  that  o[ 
tin-   binn.in   e\e.  is  often    \  i-^ibh•   \i<   the   lunnau    n.ikecl   e\  e.      In 

cats,  it  is  seen  as  a  gn-i-n  hnniiiosity. 
Tarsitis.      Inllaniination  of  the   laisus  of  the  lid. 


TARSOPLASTY  409 

Tarsoplasty.     Plastic  surgery  of  the  eyelid. 

Tarsorrhaphy.     Sewing  of  the  eyelid. 

Tarsotomy.    Cutting  of  the  tarsal  cartilages. 

Tarsus.  The  cartilaginous  framework  which  gives  shape  and 
support  to  the  eyelids. 

Tears.  The  secretion  of  the  lachrymal  glands  which  continually 
pour  into  the  eyes  through  the  lachrymal  ducts,  keeping  the 
membranes  moist  and  clean,  and  are  drained  into  the  nose 
through  the  lacrymal  sac  and  nasal  ducts. 

Svibstantially  the  tears  consist  of  slightly  salty  water. 

Teichopsia.     A  scintillation  with  a  jagged  outline. 

Telescope.  An  optical  instrument  by  which  distant  objects  are 
brought  within  the  range  of  distinct,  or  more  distinct,  vision. 
This  it  does  in  two  ways:  (1)  It  enlarges  the  entrance  aper- 
ture of  the  rays  by  substituting  its  own  objective  lens  or 
reflector  for  the  human  eye,  thus  gathering  additional  light 
from  the  object-space,  and  (2)  It  magnifies  the  image  thus 
produced. 

In  its  simplest  form,  the  telescope  consists  of  a  tube,  or  set 
of  sliding  tubes,  in  w'hich  are  an  objective  lens  and  an  eye-lens, 
both  convex.  If  the  axes  of  the  two  lenses  and  their  principal 
focal  points  (posterior  and  anterior,  respectively)  coincide, 
rays  which  are  focussed  into  an  image  by  the  objective  lens 
will  emerge  from  the  eye-lens  as  parallel  rays,  and  are  then 
focussed  by  the  eye  to  form  a  magnified  image  on  the  retina. 

There  are  two  principal  types  of  telescope,  the  refracting 
and  the  reflecting  telescope.  The  former  is  constructed  of 
lenses  only,  which,  b}'  successive  refractions,  produce  the 
desired  result.  It  is  very  essential  that  the  objective  lens,  or 
series  of  lenses,  be  as  free  as  possible  from  aberration,  and  for 
this  purpose  they  are  usually  made  of  triplex  glass,  to  correct 
this  error.  The  aberration  of  the  eye-lens  is  not  of  such 
moment. 

The  reflecting  telescope  is  composed  of  specula,  or  concave 
mirrors,  together  with  lenses,  and  is  capable  of  much  greater 


410  TELESCOPIC  VISION 

range  of  action  than  the  refracting  type,  besides  avoiding  much 
of  the  aberrati(jn  that  pertains  to  the  latter.  Xcwton,  Ciregory. 
Herschell  and  I  .ord  Rossc  all  took  a  i>art  in  its  de\"elopment, 
and  it  is  the  type  generally  used  in  astronomical  observations. 

Telescopic  Vision.  Narrowing  down  of  the  \isual  field  so  that 
the  patient  sees  as  through  a  tube,  it  occurs  in  certain  forms 
of  concentric  optic  atrophy. 

Tendon  Recession.  Mo\ing  a  tendon  of  the  eyeball  further  back 
to  remedy  stral)ismus.     The  opposite  of  advancement. 

Tendons.  The  fibrous  cords  by  which  the  extrinsic  muscles  of 
the  eyeball  are  attached  to  the  sclera.  They  have  no  power 
of  contraction,  but  merely  serve  to  transmit  the  i)ull  of  the 
muscle  to  the  eyeball. 

Tenonitis.     Intlammation  of  Tenon's  capsule. 

Tenon's  Capsule.     See  Capsule. 

Tenotomy.  Complete  or  })artial  dividing  of  the  fibres  of  the 
tendons  (see  above)  for  the  correction  of  strabismus.  It  be- 
longs to  the  province  of  the  eye  surgeon. 

Tensor  Tarsi.  .\  small  muscle  having  its  origin  at  the  crest  of 
the  lacrimal  bone  and  inserted  into  the  tarsus  at  the  imier 
canthus.  It  is  sui)i)lie(l  by  the  seventh  nerve.  By  stretching 
the  tarsus  it  compresses  the  lacrimal  jjuncta  and  sac. 

Test  Chart.  .\  chart  with  letters  or  geometrical  figures  on  it 
for  subjectiv  (Iv  testing  the  visual  acuity  or  the  refraction  of 
the  eye.  There  are  several  varieties  of  test  chart  in  use.  of 
which  the  following  are  the  ctunmonest  : 

.Snellen's  Test  Tv  pes.  These  consist  of  letters  of  varyinj.; 
size  arranged  in  rows.  (  hvv  lacli  row  is  a  number  indicating 
the  distance  at  which  each  letter  in  the  row  appears  to  a 
normal  eye  umler  a  visual  angle  of  .''  miu.  l-.ach  letter  is  in- 
scribed in  a  S(piarc  whose  sides  are  div  ided  by  partition  lines 
into  five  ecpial  parts,  so  that  each  paitial  Mpiaic  (representing 
the  detail  of  the  letter)  subtends  ;i  visual  .lui^le  of  1  deg.  in  the 


TEST  CHART 


411 


normal  e}"c  at  the  proper  clistance. — i.  e.  the  minimum  visual 
angle. 


vstONCMAirr  .  nmwoHOHM 


Qi 


1 200 


B  P  3,00 

T^3  Z  PI  70 

D'p:n:L  3  (50 
L  r  3  p'clu  40 

p   n  T  r  3  o  u~B  3   20 


j^^SSf?-— --■■"^ 


E 

1 

F  P 

2 

T  6  z 

3 

L  PE  D 

4 

~  P  E  "or  D 

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o    rZLOFZO 

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DBrpOTBO 

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11 

Test    Charts. 


The  chart  is  used  at  a  distance  of  6  meters,  or  20  feet,  at 
which  distance  the  patient  should  be  able  to  read  the  letters 
marked  for  that  distance,  in  which  case  we  record  his  visual 
acuity  as  being  6/6,  or  20/20.  If  unable  to  read  this  type,  we 
record  his  acuity  as  being  the  distance-number  of  the  type  he 
ought  to  read  divided  by  that  which  he  is  able  to  read.  Thus, 
if  at  6  meters  he  reads  only  the  8  meter  type,  we  say  his  acuity 
is  6/8.  Or,  in  feet,  if  at  20  feet  he  reads  only  the  40  feet  type, 
his  acuity  is  20/40. 

Snellen's  types  are  the  most  commonly  standard  of  all  forms 
for  distance  vision. 

Jaeger's  Test  Types.  These  are  similar  in  scientific  principle 
and  construction  to  Snellen's,  but  are  used  chiefly  in  the 
smaller  letters,  for  testing  near  vision. 

Clock  Dial  Chart.  For  testing  for  astigmatism.  It  consists 
of  a  series  of  black  lines  radiating  from  a  common  centre  at 


412  TEST  CHART 

the  ^•ariuus  angles  of  the  circle.  The  test  depends  upon  the 
fact  that  the  lines  on  the  chart  corresi)onding  to  the  patient's 
two  chief  meridians  will  be  seen,  respectively,  clearest  and 
faintest ;  and  the  cylinder  which  makes  all  the  lines  in  the 
chart  appear  e(jiially  black  will  correct  the  astigmatism. 

Pray's  Test  Types.  These  are  modifications  of  the  clock 
dial  lines  into  the  form  of  letters.  They  consist  of  a  series  of 
rather  large  letters,  each  being  made  up  of  black  strokes,  the 
strokes  being  made  at  a  different  angle  for  each  letter.  The 
two  letters  whose  angular  strokes  correspond  with  the  pntient's 
chief  meridians  are  seen  blackest  and  faintest,  respectively 

Practically  all  astigmatic  charts  are  variations  of  the  clock 
dial  chart. 

Test  charts  are  used  either  with  reflected  light  or  with 
transmitted  light — being,  in  the  second  case,  made  of  some 
translucent  material.  Thev  are  also  made  in  what  is  known 
as  "'reversed"'  form,  i.  e.  with  the  letters  or  figures  jirinted 
backward,  so  that  they  can  be  viewed  in  a  mirror  where  the 
operator's  space  does  not  permit  of  his  having  an  uninter- 
rupted 6  meters  or  20  feet  range.  For  children  and  illiterate 
persons  picture  charts  are  substituted  for  letter  charts. 

Voerhoefif's  Astigmatic  Chart  consists  of  two  circular  disks 
pivoted  through  their  centre  to  a  tlat  board.  u[)on  which 
degrees  are  marked  off.  so  that  the  disks  can  be  rotated  to  any 
desired  angle.  In  one  disk  are  drawn  rectangular  lines,  through 
which  run  two  dark  lines  at  right  angles  to  each  other  (prin- 
cipal meridians).  In  tlie  dark  disk  are  drawn  concentric 
circles,  through  which  run  dark  nicridianal  lines,  similar  to  the 
wheel  chart. 

By  the  second  disk  tlu-  axis  of  the  astigmatism  i>  tletermineil. 
by  ascertaining  the  two  chief  ineridi.inal  lines.  The  tirst  chart 
is  then  set  with  the  two  cross  lines  at  the  angles  corresponding 
with  the  chief  meridians  of  astigm.itism.  and  the  degree  of 
astigmatism  determined   by  comparing  thesi-  tw«t  cross  lines. 

Parker  has  modified  XOerhoelf's  chart  by  omitting  the  two 
cr(jss  lines  in  the  first  section,  :ind  the  coiu-entric  line>  in  the 
second. 

riiomas'  .\>tigmatic  (hart  consists  oi"  two  cross-lines,  each 
Consisting  of  three  det;iil  lines  separated  l)y   the  pro])er  \isual 


TESTS  413 

angle  distance,  which  are  made  to  revolve  inside  a  graduated 
circle.  The  chief  meridians  are  established  by  revolving  the 
cross,  and  the  astigmatism  corrected  by  making  the  two  sets 
of  detail  lines  equally  discrete  and  distinct. 

Tests.  It  would  be  impossible  to  enumerate,  far  less  to  describe, 
in  a  work  of  this  nature,  all  the  tests  employed  by  ophthalmol- 
ogists and  recorded  in  the  literature  of  the  subject  for  func- 
tional errors  of  the  eye.  We  must  be  content  to  limit  our- 
selves to  the  relatively  few  that  have,  by  common  consent, 
become  standard.  The  details  of  these  tests,  and  the  principles 
upon  which  they  are  based,  will  be  found  in  various  parts  of 
this  book,  under  the  headings  to  which  they  respectively 
belong.  In  this  place  all  that  will  be  given  is  a  brief  technical 
formulary  for  each  of  them. 

SUBJECTIVE    TESTS. 

\'isual  Acuity.  To  be  taken  as  a  preliminary  to  every 
examination.  First  with  both  eyes  in  vision,  and  then  with 
each  eye  separately,  have  the  patient,  seated  six  meters  (20 
feet)  from  the  distance  chart,  with  the  chart  under  good 
illumination,  read  the  lowest  line  he  is  able  to  read.  With  the 
number  of  this  line  as  a  denominator,  and  that  of  the  normal 
as  a  numerator,  we  express  the  visual  acuity  in  a  refraction. 
Thus,  if  the  patient  is  seated  at  a  distance  of  20  feet,  the 
normal  number  is  20;  if  he  reads  only  as  far  as  line  40,  his 
visual  acuity  is  recorded  as  20/40.     See  Acuity,  Visual, 

Pin  Hole.  To  determine  whether  a  lens  can  improve  vision. 
Under  the  same  conditions  as  stipulated  above,  place  before 
each  eye  separately,  (the  other  eye  being  meanwhile  excluded), 
the  pin  hole  disc,  and  ascertain  if  the  patient  can  read  any 
further  down  the  chart,  or  any  clearer,  than  without  it.  If  so, 
the  vision  can  be  improved  by  a  lens ;  if  not,  the  eye  is  probably 
pathologic. 

Fogging.  An  excellent  subjective  method  for  working  out, 
at  one  and  the  same  time,  astigmatism  and  spherical  error. 

1.  With  the  chart  at  20  feet,  place  before  the  eye  a  plus  lens 
strong  enough  to  blot  out  both  letters  and  wheel-lines. 

2.  Gradually  reduce  the  plus  power  with  minus  lenses  until 
the  lines  of  the  astigmatic  wheel  just  become  visible. 


414  TESTS 

3.  If  one  of  these  lines  is  tlecidedly  blacker  than  the  others, 
find  a  minus  cylinder  which,  with  its  axis  across  the  blackest 
line,  makes  them  all  equally  clear. 

4.  Leaving  the  cylinder  (if  any)  in  place,  continue  to  reduce 
the  plus  spherical  power  with  minus  spheres  until  patient  can 
read  20/20  type. 

5.  The  net  amount  of  lens  power  represented  by  the  lenses 
before  the  eye  when  this  point  is  reached,  (including  the 
cylinder,  if  any,  is  the  patient's  distance  correction. 

If  the  patient  has  no  astigmatism,  then  of  course  item  3  will 
be  omitted,  as  there  will  be  no  apparent  difi'erence  in  the  lines 
of  the  astigmatic  chart. 

Thus,  assume  that  we  fog  with  a  plus  5  1).  Reducing  to 
4  D.  the  wheel-lines  become  faintl}-  \isible,  with  the  \ertical 
line  standing  out  clearest.  A  minus  .75  1).  cylinder,  axis  180. 
makes  them  all  equally  clear.  Leaving  the  minus,  .75  D. 
cylinder  in  place,  we  go  on  reducing  the  spherical  jjower  until 
at  plus  2  1).  patient  reads  20/20.  His  correction  is  plus  2  D. 
sph.  minus  .75  D.  cyl.  ax.  180. 

Stcnopaic  Slit.  A  good  test  in  nian\'  cases  of  astigmatism, 
especially  myoj)ic  astigmatism,  but  not  a\ailablc  in  every  case. 

1.  With  the  chart  at  20  feet,  under  bright  illumination,  and 
the  untested  eye  excluded,  place  the  stcnopaic  slit  beft)re  the 
eye. 

2.  Revolve  the  slit  until  the  angle  is  ftiund  at  which  patient 
reads  type  best.  If  this  ])e  not  20/20.  make  it  so  by  means  of 
plus  or  minus  sphere.  This  sphere  is  the  measure  and  correc- 
ti(jn  of  one  of  the  chief  meridians  cjf  astigmatism. 

3.  Revohe  the  slit  to  the  opposite  angle,  which  will  be  that 
of  w(jrst  \  ision,  and  repeat  the  process.  The  lens  which  gi\es 
20/20  is  the  mea^urt-  and  correction  of  the  other  cliief  uieriilian. 

4.  Calculate  the  ctMUpound  of  two  crt)ss-cylinder>  repre- 
sented by  the  above  corrections. 

Thus,  sup])ose.  tli.it  the  slit  gi\es  best  \  ision  at  *t)  deg., 
re(juiring  minus  2  1).  to  make  it  20  20.  .At  180  it  gives  worst 
vision,  re(|uiring  mimis  3  1).  to  m.'ike  it  20/20.  The  needed 
correction,  then,  is  two  cross-cylinders,  a  niinu^  2  1).  ax.  'H.). 
and  a  minus  3  1).  a.\.  180.  The  coinpoun<l  etpiixalent  is  minn> 
2  I),  sph.  minus  I   i ).  i\l.  a.\. 'N).      The  i)r«ii)leni  is  worked  out  in 


TESTS  415 

the  same  way,  whatever  the  findings,  always  bearing  in  mind 
that  if  one  meridian  proves  to  be  minus  and  the  other  plus,  the 
difference  between  them  is  the  sum  of  the  two  numbers. 

Chromatic.  With  a  small  circular  light  at  20  feet  distance 
and  the  cobalt  lens  before  the  patient's  eye,  proceed  as  fol- 
lows : 

Reading.  The  commonest  method  of  testing  at  reading 
distance  is  with  the  jaeger  test  types.  Find  the  closest  point 
at  which  the  proper  type  for  that  range  can  be  read :  or  place 
the  chart  at  a  certain  distance,  and  then  find  the  plus  lens 
which  will  enable  the  patient  to  read  the  proper  line  of  type. 

Lockwood's  Cross  C^vlinders.  After  finding  as  nearly  as 
possible,  by  means  of  the  foregoing  test,  the  patient's  reading 
distance,  substitute  for  the  type  one  of  Lockwood's  T  charts, 
and  place  before  the  patient's  eye  a  combination  equal  to  a 
pair  of  50  D.  cross-cylinders,  with  their  axes  coinciding  with 
the  strokes  of  the  T.  If  patient  sees  both  strokes  of  the  T 
equally  black,  the  reading  distance  is  correct ;  if  not,  add  more 
plus  lens  power  until  they  are  seen  equally  l:)lack.  The  result 
will  be  the  comfortable  reading  correction. 

OBJECTIVE   TESTS. 

Static  Retinoscopy.  With  a  plus  lens  (objective)  before  the 
eye  whose  focal  length  represents  the  distance  at  which  we 
wish  to  work,  i.  e.  a  2  D.  to  work  at  50  cm.,  proceed  as  follows : 

1.  If  no  shadow  appears,  the  eye  is  emmetropic. 

2.  If  a  shadow  is  seen  moving  with  the  mirror,  the  eye  is 
hyperopic.  Find  the  plus  lens  which  abolishes  the  shadow. 
This  lens  is  the  measure  and  correction  of  the  error. 

3.  If  a  shadow  is  seen  moving  against  the  mirror,  the  eye  is 
myopic.  Find  the  minus  lens  which  abolishes  the  shadow. 
This  lens  is  the  measure  and  correction  of  the  myopia. 

4.  If,  upon  beginning  to  shadow,  the  edge  of  the  shadow  is 
seen  to  tilt  away  from  the  vertical,  the  eye  is  astigmatic,  with 
one  of  its  chief  meridians  coinciding"  with  the  edge  of  the 
shadow.  The  eye  is  then  to  be  shadowed  across  this  meridian, 
and  in  the  opposite  direction. 

5.  If,  upon  shadowing  the  horizontal  meridian,  as  correction 
is  approached,  a  band  of  light  is  seen  to  lie  across  the  pupil  in 


416  TESTS 

that  direction,  the  eye  is  astigmatic,  and  upon  comi)letion  of 
the  horizontal  meridian,  the  \ertical  meridian  must  be  shad- 
owed in  the  same  way.    See  1.  2.  and  3. 

6.  After  shadowing  each  meridian  separately  in  an  astig- 
matic e}e,  the  correction  is  calculated  as  from  two  cross- 
cylinders  of  the  power  indicated.     (See  Stenopaic  Slit).    Or, 

7.  Having  completed  the  correction  of  one  meridian,  a  cylin- 
der of  the  gi\en  power  ma\-  be  placed  before  the  eye,  its  axis 
at  right  angles  to  the  meridian  in  cpiestion,  and  the  other 
meridian  may  then  be  shadowed  and  corrected  as  a  sphere. 

Dynamic  Retinosc()i)y.  An  excellent  method  of  finding  a 
patient's  comfortable  near  point.  With  distance  correction  on. 
the  accommodation  is  fixed  upon  the  mirror  of  the  retinoscope 
at  a  given  near  point — say  33  cm. — and  the  movement  of  the 
shadow  noted.  If  neutral,  come  closer  to  the  eye  until  it  is 
■'with,'"  then  draw  back  until  it  is  neutral  again.  The  closest 
point  at  which  neutrality  can  still  be  ()l)tained  is  the  comfort- 
able near  point. 

When,  as  sometimes  happens,  the  shadow  moxement  is 
"with"  at  all  convenient  distances,  place  extra  plus  power 
before  the  eye  and  proceed  with  the  test,  allowing  for  the  extra 
power  thus  utilized  in  prescribing  the  glasses. 

Ophthalmometry.  W  hen  the  head-rest  has  been  proi)erl\- 
adjusted,  and  the  telescope  focussed,  so  that  the  mires  are  seen 
distinctly  u|)on  the  cornea,  ])roceed  as  follows: 

1.  Tiu'ii  the  r(\(il\ing  cylinder  until  the  two  central  mires 
"line  up."  i,  v.  until  the  lines  running  through  them  are  cou- 
tiuuous  will)  each  other.     This  represents  one  chief  meridian. 

2.  By  means  of  the  thumb-screw,  bring  the  edges  of  the  two 
central  mires  together,  or  separate  tluin  if  they  oxerlaj).  until 
thev  are  just  in  contact.  This  records  on  the  dial  the  dioptric 
\alue  (jf  the  meridian  under  measurement. 

3.  Ke\()l\e  the  cylinder  to  exactly  at  right  angles  with  its 
first  position,  where  the  mires  will  "line  u))"  ai^ain.  Ibis  rep- 
resents the  other  chief  meridian. 

4.  Separate  or  biiug  togetlu-r  the  i-dm'S  ol  the  centr.il  mires 
again  until  tluy  just  touch.  This  records  on  the  dial  the 
dioptric  \;iluc  ol   the  other  meridi;»n. 


TESTS  417 

5.  The  difference  in  values  between  these  two  meridians  is 
the  measure  of  the  astigmatism.  The  angles  of  the  two  chief 
meridians  is  shown  on  the  dial. 

MUSCLE  TESTS. 
Maddox  Rod.    This  test,  which  is  used  for  infinity  only,  is 
as  follows : 

1.  Place  the  Maddox  rod  before  one  eye.  and  a  red  glass 
before  the  other.  (If  the  rod  is  red,  no  other  glass  is  neces- 
sary). 

2.  Direct  attention  to  a  round  small  light  at  infinity. 

3.  If  there  is  no  imbalance,  the  patient  sees  a  bar  of  light  and 
a  round  light,  one  red  and  the  other  white,  in  about  the  same 
lateral  plane. 

4.  If  there  is  esophoria,  the  bar  and  the  light  are  separated 
in  the  same  direction  as  the  eyes  viewing  them. 

5.  The  prism  which,  base  out,  brings  the  two  images  into 
the  same  lateral  plane,  is  the  measure  of  the  esophoria. 

6.  If  there  is  exophoria,  the  images  are  separated  in  the 
opposite  direction  (crossed  diplopia). 

7.  The  prism  which,  base  in,  brings  the  images  together,  is 
the  measure  of  the  exophoria. 

8.  By  turning  the  Maddox  rod  to  vertical,  the  bar  will  be 
seen  horizontally,  and  the  vertical  balance  similarly  tested. 

Phorometer.     This,  also  should  be  used  at  infinity. 

1.  Set  the  prisms  base  in  and  direct  patient's  attention  to 
small  light  at  infinity.    He  will  see  two  images. 

2.  If  the  two  images  are  on  same  level,  there  is  no  vertical 
imbalance. 

3.  If  not  on  same  level,  there  is  vertical  imbalance.  Turn 
the  prisms  until  they  are  on  a  level,  and  this  will  record  on 
the  index  the  amount  of  imbalance. 

4.  Turn  prisms  base  up  and  base  down,  respectively.  Images 
are  now  doubled  vertically. 

5.  If  the  images  are  in  same  vertical  plane,  there  is  no  lateral 
imbalance. 

6.  If  not  in  same  vertical  plane,  there  is  lateral  imbalance. 
By  turning  the  prisms  until  they  are  brought  into  line  the  in- 
dex shows  nature  and  extent  of  imbalance. 


418  TETRANOPSIA 

Dot  and  Line.  This  is  a  finer  test  and  therefore  more  suited 
for  near  test. 

1.  Placing  the  dotted  hne  horizontally  before  the  patient, 
at  reading  distance,  put  a  6  dioptre  prism,  base  up,  before  the 
right  eye.  This  doubles  the  line  and  dot,  so  that  he  sees  two 
lines  underneath  each  other. 

2.  If  the  dots  on  the  two  lines  (true  and  false)  are  imme- 
diately underneath  each  other,  there  is  no  lateral  imbalance. 

3.  If  the  lower  dot  is  shifted  to  the  right  of  the  upper,  there 
is  esophoria,  to  be  measured  by  the  prism,  base  out;  which 
brings  them  into  vertical  line  with  each  other. 

4.  If  the  lower  dot  is  shifted  to  the  left,  there  is  exophoria  to 
be  measured  by  the  prism,  base  in,  which  lines  them  up. 

5.  Turn  the  line  to  the  vertical,  and  produce  double  vision 
with  a  pair  of  6  dioptre  prisms  base  in.  Test  the  vertical  bal- 
ance in  a  similar  manner. 

Adduction.  With  the  eyes  fixing  a  point  of  light  at  infinity, 
gradually  add  prism  power,  base  out,  until  the  image  doubles. 
The  amount  of  prism  power  with  which  single  vision  is  main- 
tained represents  the  patient's  power  of  adduction. 

It  is  important  that  the  prism  power  be  added  to  one  eye 
only  at  first,  until  several  dioptres  are  reached,  and  thereafter 
in  equal  amounts  to  each  eye,  so  that  there  will  always  be 
considcrabl}-  more  power  before  one  eye  than  before  the  i)tlier. 

Abduction.  Pursue  exactly  the  same  course  as  in  adduction, 
except  that  the  prism  power  must  be  applied  base  in. 

Tetranopsia.    Diminution  of  the  vision  l)y  one-f(.)urlh. 

Tetraophthalmos.     A  monster  with   four  eyes. 

Thalmus.     A  large  ganglion  in  tin-  nptic  tract. 

Thrombosis.  The  gradual  Idling  up  of  tlu-  lunun  of  an  artery 
by  accuinulation  of  fibrous  deposits.  Wlu-n  this  occvns  in  the 
central  retinal  artery  there  is  ,i  slowly  de\eK»ping  anemia  and 
blindness  of  the  retina. 


THYROID  419 

Thyroid.  A  large  gland  situated  behind  the  sternum  and  in 
front  of  the  trachea.  Its  only  importance  to  the  ophthalmol- 
ogist is  that  in  certain  diseases  of  the  gland  there  is  a  bulging 
of  the  eyeballs  from  their  sockets,  and  certain  other  ocular 
symptoms.    See  Exophthalmic  Goitre. 

Tiedemann's  Nerve.  A  nerve  that  enters  the  eye  along  with  the 
optic  nerve. 

Tinea  Tarsi.    Same  as  Blepharitis. 

Tobacco  Amaurosis.  Reduction  of  vision  due  to  poisoning  of 
the  retina  by  nicotine  from  smoking  excessively.  It  is  usually 
in  both  eyes  equally,  and  characterized  by  central  scotoma. 
Vision  is  better  at  night.  Abstinence  from  tobacco  usually 
effects  a  cure. 

Tonic.  This  word  is  applied  to  a  muscle  contraction  that  per- 
sists steadily,  without  intermission,  as  distinguished  from  a 
clonic  spasm,  which  makes  and  breaks  rhythmically.  An 
example  of  tonic  muscle  spasm  is  seen  in  the  ciliary  muscle 
in  latent  hyperopia. 

Tonometer.  An  instrument  for  measuring  the  tension  of  the  eye- 
ball.    It  belongs  to  the  ophthalmic  surgeon. 

Toric.  Torus.  A  form  of  lens  surface  which  combines  a  sphere 
with  a  cylinder. 

Toric  Lens.    See  Lens. 

Toxic  Amblyopia.  Loss  of  vision  due  to  poisoning  of  the  retina 
through  the  blood-stream.    See  Amblyopia. 

Trachoma.  A  highly  contagious  inflammation  of  the  conjunc- 
tiva, characterized  by  sago-like  granulations,  which  greatly 
thicken  the  eye-lids  and  irritate  the  cornea,  often  to  the  pro- 
duction of  growths  on  the  later  (pannus).  Its  diagnosis  and 
treatment  belong  to  the  ophthalmic  surgeon,  to  whom  the  case 
should  be  hurried  the  instant  the  disease  is  suspected.    Mean- 


420  TRACT 

time,  the  patient  should  be  isolated,  so  far  as  contact  with 
other  people  is  concerned,  and  every  prophylactic  and  hygienic 
measure  taken  for  his  own  and  other  people's  safety. 

Tract,  Optic.    See  Optic  Tract. 

Transillumination,  The  shininj^  of  light  through  a  translucent 
medium.  The  inspection  of  a  part  or  organ  by  means  of  strong 
light  transmitted  through  it. 

Transit.  Passing  across.  In  optics  the  word  denotes  the  move- 
ment of  the  light  area  in  retinoscopy. 

Translucency.  The  property  of  transmitting  light,  without  being 
transparent.  Frosted  glass  is  translucent,  as  it  transmits  light, 
but  we  cannot  see  through  it. 

Translucent.     Transmitting  light,  but  not  transparent. 

Transparency.  The  property  of  transmitting  light  without 
changing  its  character,  so  that  one  can  see  through  a  trans- 
parent body. 

Transposition.  Changing  the  form  of  a  compound  lens  without 
changing  its  optical  value. 

In  changing  compound  spheres  and  cylinders,  the  following 
rules  apply : 

The  algebraical  sum  of  the  sphere  and  cylinder  will  give 
the  i)ower  of  the  new  sphere. 

The  power  of  the  cylinder  remains  the  same,  but  its  sign  and 
axis  are  rcNcrsed. 

In  transposing  cross  cylinders  It)  a  comi)i)und  sphere  and 
cyliiuler  the  following  rules  apply: 

Use  the  power  of  one  cylinder  for  the  sphere. 

Use  the  algebraical  difference  between  the  two  cylinders 
for  the  new  cylinder,  with  its  axis  the  same  as  the  other  cyl- 
inder. 

In  transposing  the  compound  sphere  and  cylinder  into  cross 
cylinders  observe  the  folknving  rules: 


TRIAL  CASE  421 

Use  the  power  of  the  sphere  for  one  of  the  cross  cylinders, 
with  its  axis  at  right  angles  to  the  cylinder  in  the  compound. 

Use  the  algebraical  sum  of  sphere  and  cylinder  for  the  new 
cylinder,  with  its  axis  the  same  as  that  in  the  compound. 

Trial  Case.     A  case  containing  the  necessary  lenses  and  instru- 
ments for  making  subjective  tests  of  vision  and  refraction.     A 
well  equipped  case  should  contain : 
30  pairs  convex  spheres 
30  pairs  concave  spheres 
20  pairs  convex  cylinders 
20  pairs  concave  cylinders 
10  prisms 
1  Plain  red  glass 
1  Opaque  disc 
1  Stenopaic  slit 
1  Ground  glass  disc 

1  Maddox  Rod  Disc 

2  or  3  Smoked  glasses  of  different  shade 
2  Trial  frames,  3  and  2  cell  respectively. 

The  use  of  the  trial  case  and  its  contents  is  described  under 
the  various  tests. 

Trichiasis.  Scratching  of  the  cornea  by  the  eyelashes.  See 
Entopion. 

Trichitis.    Inflammation  of  the  roots  of  the  eye  lashes. 

Trichromatic.    Having  three  colors. 

Trifocal.  In  some  cases  it  is  desirable  to  furnish  a  presbyopic 
patient  with  three  focussing  points,  in  which  case  three  differ- 
ent focal  curves  are  ground  upon  the  lens,  or  two  wafers,  of 
differing  focal  curves,  cemented  on  to  the  original  lens.  Such 
lenses  are  said  to  be  tri-focal. 

Trigeminus.  A  name  given  to  the  fifth  cranial  nerve,  because  of 
its  three  branches.  One  branch  is  the  ophthalmic,  which  fur- 
nishes sensation  to  the  eyeball  and  lids. 


422  TRIPLOPIA 

Triplopia.  Seeing  three  images  of  one  object.  Usually  due  to 
a  coml^ination  of  heterophoria  or  heterotropia  and  an  irregular 
astigmatism,  i.  e.  a  combination  of  a  monocular  and  a  binocular 
diplopia. 

Trochlearis.  The  superior  oblique  muscle  of  the  eye,  so  called 
because  it  passes  through  a  pulley. 

Tropometer.  An  instrument  for  measuring  the  degree  of  torsion 
of  the  eyeball. 

Tunic.  A  coat,  or  container,  of  the  eyeball.  The  eye  has  three 
such  tunics,  namely,  (1)  the  retina,  the  ner\ous  tunic;  (2)  the 
chorioid,  or  vascular  tunic,  giving  rise  anteriorly  to  the  ciliary 
body  and  the  iris ;  and  (3)  the  sclerotic,  or  fibrous  tunic,  into 
which  is  set  the  cornea  anteriorly. 

Tutamina  Oculi.  Literally,  the  things  that  keep  the  eye  safe. 
Applied  to  the  protecting  appendages  of  the  eyeball,  namely, 
the  eyelids,  the  lashes,  the  eyebrows,  etc. 

Tylosis.    Thickening  of  the  lids,  due  to  ulceration. 

Typhlology.    The  science  of  blindness. 

Typhlosis.     Blindness. 

Ulcer.    A  superficial,  suppurating  lesion. 

Ultra-Violet  Rays.  Those  rays  contained  in  a  beam  of  light 
whose  wave-lengths  are  shorter  and  whose  wave-frequencies 
are  higher  than  violet  waves,  and  which  are  therefore  invisible 
to  the  human  eye.  The  term  is  often  erroneously  used  to  in- 
clude those  high-frequency  electric  radiations  with  whosv 
existence  and  use  c\en  the  layman  is  so  familiar,  such,  for  in- 
stance, as  the  X-rays.  Strictly  speaking,  it  applies  only  to 
those  high-frec|U(.'ncy  waves  which  are  (.(Hitaini'd  in  a  beam  of 
white  light. 

The   ultra-violet   rays  do   not   ha\e   any   paiticular   working 
interest  for  the  refraelionist.  except  insofar  as  it  may  be  desir- 


UMBRA  423 

able  to  prevent  them  from  entering  the  eye.    This  phase  of  the 
subject  will  be  found  discussed  under  Actinic  Rays. 

Umbra.    A  shadow. 

Umbo.  Apex.  The  umbo  of  a  lens  is  identical  with  its  pole, 
i.  e.  the  extreme  point  of  its  elevation  or  depression. 

Undulation.    A  wave-motion. 

Undulation  Theory.  The  theory,  propounded  by  Huygens,  that 
light  consists  of  waves  in  ether,  as  opposed  to  Newton's  cor- 
puscular theory.     (See  Light). 

Uniaxial.     Having  but  one  axis. 

Uniocular.    Pertaining  to  one  eye  only. 

Unit.  In  arithmetic,  the  least  whole  number,  or  1.  All  other 
numbers  are  assemblages  or  dividends  of  1.  In  physics  and 
mathematics  a  unit  is  a  known  determinate  quantity  by  the 
constant  repetition  or  division  of  which  any  other  quantity  of 
the  same  kind  is  measured. 

Unit  of  Refractive  Index  is  the  index  of  air,  which  is  numer- 
ically known  as  1. 

Unit  of  Lens  Dioptrism  is  the  lens-power  necessary  to  focus 
paraxial  waves  in  a  distance  of  1  meter,  known  as  1  dioptre. 

Unit  of  Curvature  is  the  curvature  developed  by  a  sphere 
with  a  radius  of  1  meter,  known  as  1  meter  curve. 

Unit  of  Prism  Deviation  is  a  linear  deviation  equal  to  one- 
one-hundredth  of  the  distance  at  which  the  deviation  is  meas- 
ured, e.  g.  1  cm.  of  deviation  in  1  meter  distance,  known  as  1 
prism  dioptre. 

Unit.  Angstrom's.  The  unit  of  measurement  of  the  wave- 
lengths of  the  spectrum,  estabhshed  by  Angstrom.  It  is  ten 
millionths  of  a  millimeter.  This  unit  is  seldom  used  in  optics, 
the  wave-lengths  of  visible  light  being  comparatively  large, 
and  usually  expressed  in  microns.  Physicists  who  deal  with 
high-frequency  radiations  employ  the  Angstrom  unit. 


424  UPRIGHT  VISION 

Upright  Vision.  The  projection  of  an  image  by  the  brain  in  an 
upright  position,  in  spite  of  the  fact  that  the  retinal  image  is 
inverted.  Many  ingenious  theories  have  been  propounded  to 
account  for  this  phenomenon.  The  real  explanation  probably 
is  that  the  projection  of  the  stimulus  by  the  brain  reaches  back 
to  the  place  where  the  light  enters  the  eye,  i.  e.  the  cornea ;  so 
that  what  the  brain  sees  is  not,  strictly  speaking,  the  retinal 
image,  but  the  corneal  image,  modified  by  the  refracting 
media  and  the  retina. 

Uvaeformis.     The  middle  coat  of  the  choroid. 

Uvea.  A  name  given  to  the  entire  tissue  system  pertaining  to 
the  second  or  vascular  tunic  of  the  eye,  namely,  the  choroid, 

the  iris,  and  the  ciliary  body,  as  a  whole.     It  is  also  called  the 
uveal  tract. 

Uveal.    Pertaining  to  the  uvea. 

Uveitis.  Infection  and  inflammation  of  the  entire  uvea  (See 
above).  As  the  uveal  structures  all  belong  to  the  same  tunic, 
they  are  frequently  all  involved  in  similar  diseases. 

V.     Abl)reviation  for  vision. 

Vaso-Motors.  A  name  given  to  the  system  of  muscles  and 
ner\  cs  supplied  to  the  walls  of  the  arteries  (especially  the  small 
arterioles)  and  capillaries  of  tl\e  body,  whose  stimulation 
causes  these  vessels  to  dilate  or  contract,  as  the  case  may  be, 
thus  permitting  an  increased  flow  of  blood  to  the  area,  or  driv- 
ing the  blood  out  of  it.  The  \aso-mi)tors  have  both  a  general 
systemic  action  and  a  local  action. 

GENERAL  ACTION. 
The  general,  systemic  function  of  the  vaso-motors  is  to 
maintain,  throughout  the  arterial  system,  the  minimum  blood 
pressure  which  is  essential  to  life  and  well-being.  The  reflex 
centre  which  controls  the  vaso-motor  system  is  located  in  the 
medulla  oblongata,  and,  like  all  the  ci'iitris  in  the  nu-dulla,  is 


VENAE  VORTICOSAE  425 

an  automatic  centre,  which,  however,  is  subjected  to  influence 
by  the  brain.  In  health,  and  under  ordinary  conditions,  this 
centre  keeps  a  constant  flow  of  nerve  energy  to  the  vaso- 
motors, thus  maintaining  a  uniform  minimum  tone  in  the 
muscles  of  the  vessels,  which,  in  turn,  maintain  a  uniform 
minimum  blood  pressure.  Occasional  physiologic  variations, 
up  and  down,  are  produced  by  powerful  mental  emotions — 
anger,  surprise,  fear,  etc. — but  are  rapidly  equalized. 

LOCAL  ACTION. 

The  local  function  of  this  important  system  is  twofold : 

1.  It  insures  adequate  secretions  at  such  points  in  the  body 
as  demand  them,  by  dilating  the  vessels  at  these  points  and 
sending  an  increased  blood  flow  to  the  secretory  glands. 
Wherever  increased  glandular  activity  is  temporarily  needed — 
as  at  the  stomach  and  small  bowel  during  digestion — there,  by 
an  automatic  reflex,  the  vaso-motors  dilate  the  blood-vessels, 
and  flood  the  glands  with  stimulating  blood. 

2.  It  equalizes  the  blood  pressure  in  various  parts  of  the 
body  which  would  otherwise  be  disturbed  by  local  differen- 
tials. Thus,  when  the  splanchnic  vessels  are  dilated  during 
digestion,  the  dilated  vessels  are  filled  with  blood  at  the  ex- 
pense of  the  vessels  of  the  skin  and  muscles.  The  pressure 
in  the  latter  vessels  would  fall  below  the  minimum  if  it  were 
not  for  the  automatic  action  of  the  vaso-motors,  which  contract 
the  skin  and  muscle  vessels,  thus  maintaining  the  blood  pres- 
sure. 

The  nerves  and  muscles  which  dilate  the  vessels  are  known 
as  vaso-dilators ;  those  which  constrict,  as  vaso-constrictors. 
There  is  some  dispute  among  physiologists  as  to  the  actual 
existence  of  vaso-dilators,  many  holding  that  vaso-dilation  is 
the  result  of  inhibition  of  the  vaso-constrictors. 

Venae  Vorticosae.    Veins  of  the  eyeball.      See  Veins. 

Vergence.     The  turning  of  the  eyeball. 

Vernal  Conjunctivitis.     Commonly  called  Spring  Catarrh  of  the 
Eyes.      An   infectious   type    of    conjunctivitis    which    attacks 


426  VERTEX 

children's  eyes  particularly  in  the  spring-time  and  early  sum- 
mer. 

Vertex.  Properly  speaking,  the  vertex  of  a  lens  is  the  point 
where  its  surface  cuts  the  principal  axis,  and  it  therefore  has 
an  anterior  and  a  posterior  vertex.  In  ordinary  optical  par- 
lance, however,  unless  otherwise  specified,  the  term  is  used 
in  relation  only  to  the  posterior  vertex,  i.  e.  the  point  where 
the  posterior  refracting  surface  cuts  the  principal  axis. 

Vertex  Refraction.  The  vertex  refraction  of  a  lens  is  that  power 
which,  at  the  posterior  pole  of  the  lens,  i.  e.  the  posterior  ver- 
tex, will  focus  paraxial  rays  of  light  at  the  principal  focus. 
There  is  but  one  form  and  position  of  lens  in  which  the  vertex 
refraction  is  ecjuivalent  to  its  true  or  nominal  refracting  value, 
viz..  a  piano-spherical  lens  with  its  curved  surface  posterior. 
In  this  case  all  the  refracting  is  done  by  the  posterior  surface, 
so  that  the  nominal  and  vertex  refraction  of  the  lens  are  iden- 
tical. In  any  other  form  and  position,  the  refraction  is  divided 
between  the  two  surfaces;  and  while  this  does  not  alter  the 
true  or  nominal  power  of  the  lens,  or  change  its  focal  length, 
the  values  of  the  two  refracting  surfaces  are  separated  by  a 
distance  depending  upon  the  thickness  of  the  lens  (the  distance 
between  the  two  surfaces)  and  the  refractive  index  of  the  lens- 
substance.  The  two  points  of  separation  (known  as  the  prin- 
cipal points)  lie  within  the  lens;  the  effect,  therefore,  is  to 
lengthen  the  distance  between  the  anterior  surface  and  the 
posterior  principal  focus  and  to  shorten  the  distance  between 
the  vertex  and  the  posterior  principal  focus,  thus  making  the 
vertex  refraction  greater  than  the  nominal. 

Since  the  location  of  the  two  principal  i)oints  is  a  joint  func- 
tion of  the  thickness  of  the  lens,  its  refractive  index,  and  the 
radii  of  curxature  of  its  two  surfaces,  it  is  possible,  by  a  proper 
adjustment  of  these  quantities,  to  make  the  Vertex  power  and 
the  nominal  power  equal ;  and  this  is  supposed  to  be  done  in 
the  higher  power  lenses  of  a  trial  case. 

The  same  principle  applies  to  lenses  in  series,  the  refractive 
value  of  the  last  posterior  surface  «.>f  the  series  being  the  vertex 


VERTICAL  427 

refraction  of  the  series.  This  is  the  principle  of  correction  of 
refractive  errors  of  the  eye  by  means  of  lenses.  When  we 
place  a  lens  before  the  eye,  it  and  the  eye  become  lenses  in 
series ;  and  it  is  not  that  a  plus  or  minus  lens  increases  or 
decreases  the  dioptric  power  of  the  eye,  but  that  it  shortens 
or  lengthens  the  vertex  focal  distance,  or,  in  other  words, 
moves  the  focal  plane  nearer  to  or  further  from  the  vertex  of 
the  series  (the  posterior  surface  of  the  crystalline  lens),  that 
effects  the  correction  we  are  seeking.     (See  Lens). 

Vertical.  The  specific  application  of  this  term  in  physiologic 
optics  is  to  the  extrinsic  muscles  of  the  eyes  and  their  action. 
The  vertical  muscles  are  the  superior  and  inferior  rectus 
muscles,  whose  action  is  to  move  the  eyeball  vertically  upward 
and  downward,  respectively.  Vertical  imbalance  is  an  im- 
balance of  these  muscles.  Vertical  strabismus,  a  squint  in 
which  the  eye  is  pulled  up  or  down  by  one  or  other  of  the 
vertical  recti,  and  vertical  diplopia  is  double  vision  where  the 
false  image  stands  vertically  above  or  below  the  true  image, 
due,  of  course,  to  dis-function  of  one  of  the  vertical  rectus 
muscles. 

Vertigo,  Ocular.     Dizziness  caused  by  defective  vision. 

Virtual  Focus.  An  imaginary  focus,  arrived  at  by  projecting 
the  rays  of  a  divergent  wave  backward  to  the  point  from  which 
they  appear  to  have  originated.     (See  Focus). 

Visibility.  The  quality  of  being  perceptible  to  the  vision  by 
means  of  light  communication.  The  visibility  of  an  object 
depends  upon  its  ability  to  absorb  some  of  the  light  waves 
which  strike  it,  and  to  reflect  others.  If  it  absorbed  all,  it 
would  be  merely  an  area  of  dark ;  if  it  reflected  all,  it  would  be 
simply  an  area  of  light.  By  proportioning  the  two,  it  becomes 
visible  as  an  object. 

Vision.    The  faculty  of  seeing. 
Double  vision.    See  Diplopia. 
Monocular  vision.     Seeing  with  one  eye  only. 
Binocular  vision.     Seeing  with  both  eyes  at  once. 


428  VISUAL 

Visual.    Pertaining  to  vision. 

\'isual  angle.     See  Angle. 

Visual  axis.    See  Axis. 

Visual  field.     See  Field. 

Visual  purple.  A  photo-chemical  substance  in  the  rods  of 
the  retina,  whose  dissociation  by  the  action  of  light  sets  up 
visual  stimulation  of  the  retina.  (See  Physiology  of  Vision  and 
Retina). 

Visual  Acuteness.     See  Acuity. 

Visual  Judgments.  This  is  the  name  given  in  physiology  to  cer- 
tain judgments  exercised  by  the  brain  on  the  data  supplied  it 
by  the  visual  faculty;  such  judgments  are  not  really  a  part  of 
the  faculty  of  vision,  but  are  so  intimately  connected  with  it 
as  to  be  generally  regarded  as  belonging  to  it.  They  are  as 
follows : 

Judgment  of  distance.  Arrived  at  by  a  comparison  of  the 
size  of  the  object  with  its  size  at  some  known  distance ;  by  the 
degree  of  accommodation  and  convergence  necessary  to  view 
it,  when  it  is  within  infinity.  Outside  of  infinity,  judgments  of 
distance  are  very  difficult  to  make. 

Judgment  of  size.  By  comparing  the  size  of  the  image 
(visual  angle)  with  that  made  by  other  objects  of  known  size 
and  distance. 

Judgment  of  solidity  or  third  dimension.  See  Binocular 
Vision. 

Vitreous.  Literally,  like  glass.  The  name  given  to  the  humor 
which  fills  the  posterior  part  of  the  eye  ball,  back  of  the  cry- 
stalline lens  to  the  retina.  So-called  because  it  has  resemb- 
lance to  glass.  In  fetal  life  an  artery,  the  hyaloid  artery,  a 
branch  of  the  retinal,  i)ierces  the  middle  of  the  vitreous  from 
retina  to  lens,  but  after  birth  the  artery  disappears,  leaving 
()u\y  the  canal  in  which  it  lay,  the  hyaloiil  canal.  (\"casionally 
the  hyaloid  artery  i)crsisls  in  cxtra-uteriiu'  life.  The  vitrc«.)us 
forms  one  of  the  refracting  nu-dia  of  the  eye.  its  index  of 
rcfractio!!  being  l..^.V  about  the  same  as  the  acjueous  humor. 


VON  GRAEFE'S  SIGN  429 

Von  Graefe's  Sign.  The  failure  of  the  eyelid  to  follow  the  down- 
ward movement  of  the  eyeball  in  goitre. 

Wall-Eyed.  An  expression  applied  to  a  person  whose  eyes  give 
the  impression  of  being  immobile  and  staring.  There  are  sev- 
eral causes  for  such  an  appearance,  the  commonest  of  which 
are  divergent  strabismus  and  amblyopia. 

Warm  Colors.  The  colors  lying  towards  the  red  end  of  the  spec- 
trum. 

Wave  Theory.  The  theory,  put  forward  by  Huygens,  that  light 
consists  of  a  disturbance  in  luminous  ether  which  propagates 
itself  in  the  form  a  spherical  waves.  It  is  at  present  the  ac- 
cepted theory,  on  which  the  science  of  optics  is  based.  See 
Light. 

Weiss'  Test  Types.  A  series  of  types  arranged  in  intervals  of  a 
tenth  visual  acuity,  to  facilitate  the  determination  of  equal 
intervals  of  visual  acuity  without  altering  the  distance  at 
which  the  test  is  made.  \\'eiss,  however,  does  not  approve 
of  the  method  of  expressing  visual  acuity  in  decimal  fractions. 

Wernicke  Pupillary  Reaction.  A  test  applied  to  the  light  reflex 
of  the  eyes  in  case  of  hemianopsia,  in  order  to  determine 
whether  the  lesion  is  in  front  of  or  behind  the  optic  thalamus. 
If  it  lies  in  front,  there  is  no  pupillary  response  to  light  thrown 
on  the  blind  part  of  the  retina ;  if  behind,  the  reflex  is  intact. 

Williams'  Lantern  Test.  A  test  for  color  blindness  by  means  of 
colored  lanterns.     See  Color  Blindness. 

Wink.  The  quick  closing  and  opening  of  the  eyelid.  Ordinarily 
this  act  is  performed  unconsciously,  every  few  seconds,  for  the 
purpose  of  spreading  the  lubricant  secretions  of  the  eye  over 
the  globe,  and  sweeping  away  particles  of  dust.  It  is  a  true 
automatic  reflex,  brought  about  by  irritation  of  the  fifth  nerve 
fibres  in  the  eyeball.  (See  Reflex).  Occasionally  the  reflex  is 
due  to  retinal  irritation  of  the  optic  nerve,  as  in  bright  light. 


430  WINKER 

Winker.    A  common  name  lor  the  eyelash, 

Wollaston  Prism.  The  prism  used  in  the  make-up  of  the  Javal 
ophthahnometer,  consisting  of  two  quartz  prisms  so  cut  that 
when  in  position  the  base  of  each  is  at  the  apex  of  the  other 
and  the  optical  axis  of  each  is  at  right  angles  to  that  of  the 
other  and  to  the  axis  of  vision.    See  Ophthalmometer. 

Wool  Test.  A  test  for  color  blindness  in  which  colored  wools 
are  used  as  objects.     See  Color  Blindness. 

Word  Blindness.    See  Mind  Blindness. 

Worsted  Test.    Same  as  wool  test. 

Worth's  Amblyscope.    See  Amblyscope. 

Xanthelasma.  A  Hat,  sulphur-yellow  tumor  which  occurs  on 
the  eyelid,  usually  at  the  inner  canthus,  projecting  a  little 
above  the  skin  of  the  lid. 

Xanthocyanopia.  Inability  to  pcrcci\e  red  and  green,  the  color 
perception  being  limited  to  yellow  and  blue. 

Xanthopane.    A  condition  in  which  all  objects  appear  yellow. 

Xanthopsia.     Same  as  Xanthophane. 

Xeroma.    Abnormal  dryness  of  the  conjunctiva. 

Xerophthalmia.     Conjunctivitis  with  dryness  and  atrophy. 

Xerosis.    Abiujrmal  dryness  of  the  eyes. 

X-Rays.  Also  called  Roentgen  rays,  after  their  (lisct)\  erer,  W'il- 
liclm  Conrad  Roentgen.  They  are  in\isible  radiations  trans- 
mitted through  the  ether  in  a  manner  similar  to  light,  ant! 
consist  of  \ery  short,  irregular,  non-harmonic,  electro-magnetic 
pulsations,    capable    of    passing    throuj^h    opaque    substances 


YELLOW  SPOT  431 

approximately  in  inverse  proportion  to  the  atomic  mass  of  the 
material.  They  are  produced  by  passing  uni-directional  elec- 
tric current  of  from  twenty  to  a  hundred  thousand  volts  tension 
through  a  specially  constructed  vacuum  tube,  within  which 
cathode  rays  from  the  surface  of  a  concave  cathode  are  fo- 
cussed  upon,  and  bombard,  a  target  of  refractory  material, 
such  as  tungsten,  platinum,  etc.,  from  which  focus  the  Roent- 
gen rays  radiate  in  all  directions  in  accordance  with  the  law 
of  inverse  squares.  Roentgen  named  them  X-rays,  because  of 
the  lack  of  knowledge  as  to  their  exact  nature. 

Yellow  Spot.  The  most  sensitive  spot  in  the  retina.  Anatomi- 
cally, it  lies  a  little  to  the  temporal  side  of  the  centre  of  the 
..retina,  and  is  distinguished  under  the  ophthalmoscope  by  being 
lighter  in  color  than  the  surrounding  retina,  owing  to  lack  of 
vessels  and  pigment.  Physiologically,  it  is  the  centre  of  atten- 
tive vision,  only  those  parts  of  the  image  which  fall  upon  the 
yellow  spot  being  observed  with  attention  and  detail.  It  con- 
tains no  rods,  but  cones  only,  and  no  visual  purple.  Optically, 
the  yellow  spot — or,  rather,  the  fovea  centralis,  the  central 
point  of  the  yellow  spot — is  situated  at  the  end  of  the  visual 
axis  in  the  image-space.     See  Retina. 

Young-Helmholz  Theory.  The  theory  of  color  perception  which 
holds  that  it  depends  upon  the  stimulation  of  three  retinal  ele- 
ments, whose  stimulation  corresponds,  respectively,  to  sensa- 
tions of  red,  blue  and  green.    See  Color. 

Zeiss'  Glands.  The  sebaceous  glands  situated  at  the  free  margins 
of  the  eyelids. 

Zinn's  Ligament.  A  circular  ligament  at  the  optic  foramen, 
attached  to  the  bones  of  the  orbit,  with  a  circular  opening 
through  which  the  optic  nerve  passes  into  the  globe,  and  from 
which  arise  the  rectus  muscles  of  the  eye. 

Zone  of  Zinn.  A  circle  of  vessels  around  the  optic  nerve  where 
it  pierces  the  eyeball,  which  supply  nutrition  to  the  substance 
of  the  optic  nerve  itself. 


432  ZONULA 

Zonula.  Literally,  a  little  zone.  The  zonula  of  the  eye  is  the 
thin  membraneous  ligament  which  holds  the  crystalline  lens 
in  place.  It  is  also  known  as  the  suspensory  ligament.  It  is, 
in  fact,  a  part  of  the  ciliary  body,  from  which  it  takes  its  rise, 
dividing  and  passing  over  the  edge  of  the  lens,  forming  its 
anterior  and  posterior  suspensory  ligaments,  which  are 
attached  to  the  capsule.  The  triangular  space  between  the 
zonula  and  the  lens  is  called  the  canal  of  Pettit. 

Zonule  of  Zinn.    Same  as  Zonula. 

Zonulitis.     Inflammation  of  the  zonula. 


^T  -''  the  Alainecfa 

■       -  t;iOIl 

pu.metrists 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

OTTc: :::  ~r> ;  ^.^_  .  ,. 

This  book  is  due  on  the  last  date  stamped  below,  or 

on  the  date  to  which  renewed. 

Renewed  books  are  subjea  to  immediate  recall. 


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LI)  Zl-SOf/i-S.'ii? 
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General  l.ihr.iry 

University  of  Ciilifornia 

Berkeley 

U.C,  BERKELEY  LIBRARIES 


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